All of my college suite-mates are obsessed with Mario Kart, so I made them this hat for christmas. Even though this tutorial covers how to make your own Blue Shell hat, you could easily swap the blue fleece for red or green to make the Red or Green Shells, or even use the basic construction style to make a Mushroom hat! (if you do, please sent me pics! I'd love to see that).
This project took me about 16-18 hours over the course of 4 days.

Materials:

White, blue, and black thread
1 yard each of White, blue, and back fleece or felt (I used fleece)
A fiber-based filler (can be found at craft stores)
1" elastic band

Tools:

Sewing machine
Pins
Needle
Scissors
Pen or Sharpie
Measuring tape
Time and patience

NOTE: Almost all of the sewing in this project is done with a sewing machine using a straight stitch. When it's absolutely essential to hand sew, I explicitly say so, but in all other places where I just say "sew" you can assume that means on the machine, with a straight stitch, making sure to add several leading and tailing stitches so your whole seam doesn't unravel. If you dont have a machine, I will say it is possible to do this project completely by hand, but it may take you several years.

Step 1: Design & Prototyping

The first step is always design and prototyping. If you dont want to make the blue shell hat, find some concept art for the hat you want to make. I've included the photo I used as reference (above).

What We Want:

The basics of the hat is a 7 hexagon pattern: one regular hexagon on top and 6 'elongated' hexagons around the sides, each with a white horn sticking out from the center. The hexagons have to be elongated because regular hexagons tile to make a flat surface, and this hat is hemispherical.
6 isosceles triangles around the edges fill in the gaps between the hexagons and the white rim that goes around the whole base of the hat (I won't be making the under-belly part). In between the hexagons and triangles, there is a black lining that makes the hexagon shapes really stand out. (all of this is depicted in the pattern above).
Then of course, there are 2 wings which attach right above the two triangles on either side of the hat. And thats basically it!

Now to figure out how to make each of these pieces out of fleece...

How We're Going to Make it:

First off, I knew I wanted the hat to be very 3 dimensional, so I decided to make each hexagon and triangle section a 'pillow', meaning 2 pieces of fleece stitched on top of each other, sandwiching some fluffy filler material in-between, essentially making a pillow. The horns will be made by cutting out a section of a circle in white fleece, and then sewing it so it folds into a cone. These will be attached to the top piece of blue fleece on each hexagon, and the inside of the horn will be 'open' to the inside of the hexagon pillow (the horn doesn't have a bottom face, its just open to the inside of the pillow).

To add black lining in between each shape, I decided to sew in a folded-over piece of black fleece in each seam. A cutaway view of this whole idea is in the photos above.

The white rim around the bottom of the hat will be a long strip of folded-over white fleece with the same filler stuff inside, and it will also have an elastic band inside so the rim of the hat stretches a little. The concept art shows this rim being a bit 'wavy' as it circles around the circumference of the hat. For ease of construction, mine will just be a circle (no waviness).

The wings will also be pillows, except we will sew the two pieces of fleece together with the backside of the fleece facing out. Then we will turn the wings inside out so the nice side of the fleece faces out, and the seam looks nice and professional.
If you want, I've included an optional step to make the wings even more realistic, all it entails is basically making smaller wings in the same way, and then stitching them to the larger wings. This just gives the wing more dimension by adding an extra layer of 'feathers'.

Prototyping:

Now that we know how we are going to make everything, we need to figure out how big to make each of these pieces.
I began cutting out hexagons, taping them together, and then putting them on my head. After some time, I figured out that hexagons of side length 2.5 inches work well for an all-around, good fitting hat. The Top hexagon is just a regular hexagon, with each interior angle measuring 120 degrees. The body hexagons, however, have two different interior angles, and these angles will determine how far from flat that the hat is. If the body hexagons were regular hexagons, our hat would just be flat, but if we degrease the 120 degree angles that boarder the top hexagon, we get a shape that 'pops up' and becomes more hat like. I found that if we make those top and bottom angles 107 degrees, then the hexagons will tile to produce a pretty good hat shape (look at the photos above).

Templates are provided for the two types of hexagons you'll need for this hat. Just be sure that when you print them out the side lengths are 2.5 inches.

OK, now onto the horns! I decided I wanted horns that were about 1.5 inches tall and had a diameter of 1.5 inches. After a little algebra, I figured out the right 2D shape that would roll into a cone this size. I've also included the template for this.

We will get to the wings later. Lets start making the hat!

Step 2: Making the Horns

Trace your cone template onto the backside of your fleece. Leave a little extra material along the edges you will be stitching together. Cut out your piece, and fold it over with the backside showing. Pin it so the lines line up, then use your sewing machine to sew along the line. Dont forget to add several stitches at the beginning and end of your sewing so the ends dont unravel and fall apart. Trim the threads, and then turn the cone inside-out. Make sure its still the same size you want it, and once you are happy with it, make the other 6!

Step 3: Making the Hexagons

Looking at the diagram, you see that you need 1 top hexagon, 6 body hexagons, and 6 triangles to make the hat. However, each piece is actually 2 pieces of fleece, because you need a pocket to stuff the fluffy filler stuff into, which makes the hat a little more 3 dimensional and more interesting. So, in this step, you will be cutting out 2 pieces of fleece for every hexagon you make.

Trace the "top hex" pattern onto the backside of the blue fleece, and add extra on all sides. Cut out the hexagon, and mark and cut a 1" diameter circle on the backside of the fleece in the center. I used a spool of thread to trace the circle. Cut 4 little slits in the circle to widen it a bit and add 'tabs' which you will sew to (refer to photo above). Now, push a cone up through the circle so it sticks out on the frontside of your blue hexagon. On the backside, line up the excess material on the bottom of the cone with the "tabs" that you've created around the hole in the hexagon. Pin the two fabrics together, and stitch along the circumference of the circle from the backside (you can use white or blue thread here).

You should now have a hexagon with a horn sticking out. Now cut out an identical hexagon, but dont put a hole in it! Line up the two hexagons with the horn-hexagon on top facing outward, and the regular hexagon underneath with the backside facing outward (this allows you to see the lines while sewing, and it will be on the inside of the hat so you won't see it anyway). Pin them together, and then sew them together, using the lines on the back of the regular hexagon as a guide. ONLY STITCH 5 SIDES. Trim your threads.

There should now be a sort of 'hexagon pocket'. Use the fiber-basd filler to poof-up the hexagon pocket, making sure to stuff some up into the horn to give it shape. Once you are happy with the firmness of your hexagon, sew up the last side. Trim your threads.

Now, repeat this same basic process for the 6 other 'body hexagons', the ones that are elongated. Trim your threads!

NOTE: Some fleece stretches only along one axis! To test this, just stretch your fabric along the 'x-axis' and then along the 'y-axis', If one direction stretches more then the other, then you will have to keep that in mind while cutting out your body-hexagons.
The body hexagons need to stretch in the direction indicated on the layout/pattern photo above, or the whole hat won't stretch when you pull it on your head.

Step 4: Adding Black Lining

Now we will be adding the black lines that outline each hexagon. The pattern I used can be seen in the diagram above. As you can see, for 3 of the body hexagons, the black lining will go all the way around starting at one of the top corners, and will have enough left over for one more side. The other 3 body hexagons only need lining on the lower 3 sides. This is not the only template that will outline all the hexagons of corse, but I chose it because there are only 2 distinct patterns, so you can just hammer 3 hexagons out at time using the same pattern.

Anyways, cut a 1.5 inch wide strip of black fleece that is long enough for the specific pattern you are using. Fold it over, with the frontside out, and pin it onto the correct side of your hexagon. How much of the black lining shows is really your preference, but I tried to line it up so it stuck out a little over the seam on the hexagon. Use blue thread and the sewing machine to stitch the lining on. Make sure when you are sewing that you stitch on the outside of the seam on the hexagon, so you dont end up changing the shape of the hexagon. Trim your threads!

Continue for the rest of the body hexagons (the top hexagon doesn't need lining if you use this template).

Step 5: Assemble Hexagons

Now its time to assemble your 7 hexagons. I dont think there is any best order in which to sew them together. No matter what you do, the last couple seams are going to be pretty hard to sew, just because there is going to be so much bulk getting in the way of the sewing machine.

I used a general pattern of adding one hexagon at a time, starting with the top hexagon and one body hexagon, and then one by one adding addition body hexagons around, sewing both seams (the one attaching the new hexagon to the top, and the one that attaches it to the adjacent body hexagon).

First, line up the seam by pinching the 'flaps' on the sides of the hexagon, and figure out where you want to put the seam. Then pin the flaps and sew with blue thread. Trim your threads, and continue.

Step 6: Make and Attach the Triangles

In this step you will make the triangle pillows out of blue fleece and attach them to the rest of your hat.

The reason I didn't design the shape of the triangles first is because it's really up to you how 'narrow' you make them. Obviously the two sides that border hexagons have to be the same side length as the hexagons, but the bottom side can be a range of sizes, and you should tailer the length of the bottom side to fit your head best. Making the bottom edge shorter will bring the rim of the hat in more, for a smaller head, and vise-versa.

One way you can 'size' the triangle is just go ahead and cut out 2 large pieces of felt (maybe like triangles that are 1 inch too large on all sides) and then make the pillow out of that, using the same basic steps as for the hexagons. Then, when you go to attach the triangle to the hat, sew only one side to the hexagons, and then adjust the width of the triangle by pulling the other hexagon over the triangle pillow until it looks to be the right size. Do this for all six triangles, then pin the other un-sewn seam so you can test fit the hat on your head. Adjust accordingly by changing the width of the triangles. Then go ahead and sew the other seam once the hat fits right.

At this point you should have the two sides of the triangle sewn into the hat. Now go ahead and sew some black fleece onto the bottom edge for the black lining (just like you did with the hexagons). Leave extra black fabric on each side of the seam.

Done! Now time to add the rim of the hat.

Step 7: Adding the White Shell Rim

Now it's time to make the white rim that goes along the bottom of the hat. This rim will also have elastic in it so the hat stays on your head.

Cut a 4 inch wide strip of white fleece about 30 inches long. Make sure that you cut it so the fleece stretches in the length direction (so it will stretch when you put the elastic in). Fold it over with the frontside out and pin it. Using a zig-zag stitch, sew along the edge (the zig-zag stitch will allow the fleece to stitch). Now take your elastic strap and put a cloths pin in one end. Inch by inch, scoot the pin through the white fleece 'tube' until its all the way through. Fill the tube with the fluffy filler stuff by shoving it in with a long dowel until it's spread out evenly throughout the tube. Sew up the tube by sewing through the elastic near the ends of the tube. Trim your threads!

You should now have a white tube that stretches when you pull on it.

Now its time to fit the tube to your head, and attach it to your hat!

Wrap the tube around your head and mark how long you need it. You will need about 1/2 inch to attach one end to the other to make it a loop, so consider that when you are marking the length. Also, it you want it to be tighter on your head, take a little off the length so it stretches when you pull it on.
Sew along the line you marked just like you sewed up the ends. Then cut off the excess.

If your tube length does not match the circumference of the bottom of the hat (chances are it will be shorter) then you will need to do a little extra work than just sewing the tube to the hat. Make tick marks along the tube so you have 6 equal length sections. When you go to pin the white tube along the bottom of the hat, match up each tick mark with a corresponding part on each hexagon. This will spread out the slack in the hat circumference. When you actually sew the tube on, use a zig-zag stitch and only stitch the sections where the tube borders a hexagon, leave the triangles sections un-sewn for now.

The tube should now be attached by the six sections bordering the hexagons, but should not be attached to any of the triangles.

Now is the sort-of-hard part. Chances are the triangle sections which you need to sew to the white band are going to be longer then the section of band available to sew to. Thankfully, you made the band elastic! When you go to sew this seam, stretch out the band so it's the same length as the triangle section, and then sew it with white thread with a zig-zag stitch so it can stretch and contract without breaking the thread. Trim your threads and repeat!

The idea is, when the hat is at rest, the band will be contracted to its natural length, and the sections where the hexagons meet the band will look good and won't be bunched or stretched. The sections where the band meets the triangle may be bunched, but thats ok.
When you pull the hat on your head, the elastic will stretch, and so will the hexagons and the seam because the fleece stretches and we used a zig-zag stitch. The triangles will stretch from the their bunched up state to look normal again.

Step 8: The Wings

Now it's time to make the wings!

First, do some full scale sketches of a wing and cut it out to size it to the hat. Obviously we can't make the wings to scale with the concept art, because they would literally weigh 50 pounds. The size I came up with makes the wings about half as big as they should be, but still makes for a very convincing 'Blue Shell' look. Also, these wings will be 'poofy' as well, which means we need two pieces of fleece for each wing, mirrors of each other.

Once you are happy with your wing shape, trace it onto the backside of some white fleece. As usual, cut out the fleece leaving enough room around the edges to sew effectively (1/4 inch minimum). Flip your paper template over to trace the other side of the same wing.

Once you have both fleece cutouts, pin them together so the backside of the fleece faces outward. Sew along the outlined perimeter with white thread, and be sure to leave a 2 inch section of the perimeter un-sewed near the 'bottom' of the wing (the side opposite the feather tips). Trim your threads! Now, before we turn the wing inside out, make sure to really trim away the access fleece on the outside of your stitches, especially near the feather tips. If you dont, you will see why you need to once you try to turn it inside out.

Ok, so now turn it inside out. Use a capped pen or a dowel to push out the feather tips all the way.
Plan out where you want to put your "feather creases", but dont draw on the fleece (cause this is now the frontside of the fleece). Refer to the pictures above to see exactly what I mean by "feather creases". Sew these "feather creases" with white thread and a straight stitch. Trim your threads!

Take a clump of the fluffy filler stuff and separate it out into little poofs. Shove these poofs up unto the feathers (in between the feather creases) with a capped pen via the hole you left near the bottom of the wing. You should now see your wing becoming more wing-y. Continue filling the wing until it's the desired poofiness.

Thread about 2 feet of white thread onto a hand-sewing needle, and sew up the hole in the bottom of the wing.

You are now done with your first wing! Repeat these steps to make one more (it can be exactly the same, it doesn't have to be a mirror of the first).

OPTIONAL

The wing from the concept art has more detail on the side of the wing. It looks like there are several 'layers' of feathers, not just one.
To incorporate these details, I made smaller wings with 3 feathers and sewed them onto the side of the larger wings. I used the same exact process as above, but just with a smaller template.

Step 9: Finishing Touches

At each junction of black lining, you should have one piece of fleece butt up against one continuous piece, it's time to sew these joints up!

Trim the butt end fleece so it will reach the continuous piece when stretched a little. Then thread a hand-sewing needle with 2-3 feet of black thread, and begin sewing in a helical pattern through all three sections of fleece that meet at the junction. When you are done, thread the needle through to the inside of the hat to tie the thread off. Continue with the rest of the junctions.

Now it's time to attach the wings.

Line your wings up how you want them, I put mine right over the triangles which were across the hat from each other. This is also a good time to figure out which hexagon you want to be in the front, and then choose wing placement based on that.

In terms of actually attaching the wings, I sewed the wings to the triangles with white thread and a hand-sewing needle, and tried to place my stitches under the second layer on my wing (so the stitches were mostly covered up) or in the seams in-between the feathers (again, so you can't see the stitches).

Once the wings are on, go around and trim up large excess flaps of fleece and hanging threads. Then you are done!

Step 10: Try It On!

I hope you enjoyed the tutorial! If you make this hat, or any other versions (like the red or green shells) please send me pics! I'd love to see them.

Also, please let me know if there is anything I can improve like grammar/spelling mistakes; or bigger things, like maybe I completely missed a step.

Thanks to my mom for taking several pictures when neither of my hands were free to hold the camera, and thanks to my brother for being my hat model for the photo.

Thanks again!
Sam