I decided to create this costume because the "off the rack" costume available at stores like Target and Walmart was just so bad. It was important to me to achieve the fat guy shape of Captain Underpants and to have his face look like him.

I started my research trying to figure out how to achieve the specific Captain Underpants silhouette I wanted. Luckily, I came across a video from Evil Ted Smith on youtube...How to Make Large Pattern Costumes, Tutorial.

Watch that video to understand the concept of creating a pattern. I'll go over specifics for this in step 1.

For now, here's the basic supplies for the costume:

Body

2 rolls of 24" x 36" x 1" foam
Adhesive spray
2 yards of nylon 4 way stretch knit - skin color & thread
2 yards of nylon 4 way stretch knit - white & thread

Head

Perforated window vinyl - teeth (and costume peep hole)
Hot glue gun and stick
Black spray paint & spray lacquer (for eyes)

Cape

2 yards of red acrylic felt & thread
2 jars of black fabric paint

Step 1: Creating the Pattern for the Body & Head

In this step you are creating a small version of the shape you want for your final costume as a mini-sculpture. You'll wrap that mini-sculpture in foil and then Duck tape. That foil and tape will eventually be marked, cut, peeled off and flattened to become the guides/pattern for cutting your costume foam and fabric.

View the videofrom Evil Ted Smith - How to Make Large Pattern Costumes, Tutorial. This video served as my guide. It will give you a concept of what you'll be doing in this step - with some modifications.

One example of a modification is that they use a hard block of foam (and a band saw) to create the mini sculpture. I don't have a band saw to shape hard foam so I used clay instead to mold the shape I wanted.

Materials you'll need for this step:

Clay
Aluminum foil
Duck tape
Sharpie
Exacto blade
Cutting mat (with measurements)
Long roll of white paper (2 yards of paper)

SUBTASKS

Build the mini-sculpture, and create a pattern out of it

Mold your shape until you're happy with it. Captain underpants is an egg shape.
Wrap that shaped clay with aluminum foil
Tape over that foil with Duck tape
Use a Sharpie to create a vertical guideline (with horizontal crosshairs) along the sculpture as if you were going to slice it in half vertically. The crosshairs make lining up pieces side by side more accurately when you are constructing the foam.
Because we will be building a curved shape - you also need to mark darts on the Duck tape. These darts serve a guides for recreating the curve shape from the foam and fabric.
I also marked a horizontal line all around the shape to indicate where the head and body would be split (as my costume is 2 separate pieces)
Analyze the shape and determine which vertical side of the shape has the best profile
Mark x's on the half that you don't think is perfect
Use the Exacto blade to cut into the duck tape and foil
Pull the Duck tape (not marked with x) away from the sculpture and flatten it This is your pattern.
Lay the pattern on the cutting mat - be conscious of the placement of the pattern in relation to the lines on the mat
Take a picture of the pattern on the mat making sure that the lines on the mat are square with the frame of your camera image frame.

Project the pattern, and zoom it - to trace a larger version of it

I also differed from the video in how I projected the pattern to create a larger version of it. I don't have an art projector at home. But I was able to connect my Android phone to my Android TV to cast the photo I had taken onto the TV and voila! --- I had a larger version of the pattern to trace. How big did I know how to zoom it? I measured my son estimating how large the costume needed to be on him. I figured that the egg-shaped costume should be over his head and end at his thighs. In order to get the right size pattern for you --- make sure to measure the person who will wear the costume and zoom the pattern to a measure that matches.

Using about a yard of white roll paper - trace the projected/enlarged pattern - making sure that you trace it 2x because the pattern you cut out only represents one half of the mini-sculpture. When you double the tracings - it gives you the full 3D shape again. Make sure to also copy the crosshairs - that will help you match up pieces side by side for construction.
Cut the paper pattern shapes out

Cut the foam pieces, match them up glue them back together

Lay the paper pattern shapes on the foam in an efficient manner so that you maximize the use of foam with as little wasted foam as possible.
Use a light-colored sharpie to outline the shape of the pattern onto the foam (use a light color because we want to avoid having those lines show through when you cover the foam with fabric).
Cut the foam pieces
Please give yourself time for this...and expect to do this outside or in an area that can get messy/sticky....Use adhesive spray to edges of the foam that will come together to form the shape. (watch the end of the video below to understand what I mean). I use binder clips to keep pieces together. I let my pieces dry overnight to ensure everything stuck together.
After the foam pieces dry, cut out the leg holes, arm holes (using your model to get the positioning right) and the mouth hole.

Step 2: Sew the Fabric Pieces Together

In this step you will use the paper pattern you used previously (to build the foam body and head) to cut and construct the fabric "skin" for the body and head of the costume. The photo shows the added fabric "skin" along with the underwear,

When I transferred the pattern to the fabric, I added 1/4 inch around the shape to allow for seams. For you, this will depend on how stretchy your fabric is. My fabric was very stretchy so I wound up with a "skin" that had a lot of room....leaving me to have to adjust the "skin", re-sewing to pull in fabric. Use your judgement on this. Of course it's better to have the room for error so you can adjust/correct vs. having to little fabric and your "skin" being too tight.

Materials you'll need for this step:

Adhesive spray
2 yards of nylon 2 or 4 way stretch knit & thread - skin color
2 yards of nylon 2 or 4 way stretch knit & thread - white
Fabric Pins

SUBTASKS

Lay the paper pattern onto the skin-colored fabric
Trace the pattern onto the fabric making sure to mark the crosshairs
Cut the fabric pieces out
Match up pieces and sew together

You should now have a fabric "skin" version of the body shape and head shape

Pull the body fabric over the foam body shape. Adjust the "skin" over the foam so that seam lines match.
You might try lightly spraying adhesive at this step between the foam and fabric. But you'd have to be really careful not to stain your fabric. If you don't spray it --- don't worry at all...it's not really that necessary - especially in the case that the "skin" fits loosely over the foam.

Cut holes in "skin" to match holes in body and head foam, then stich to inside of foam

Over the hole, I made a slice/cut in the fabric then pulled the edge of the fabric through the opening to the inside of the foam.
Little by little, I hand stitched the fabric to the inside of the foam - picking up a small pieces of foam and whipstitching it to a small edge of the fabric.
There were points as I was stitching that I needed to cut into the fabric - so that it would shape around a curve. Make small cuts to do this...this takes patience and eye-ing it.
Get ready to do a lot of hand stitching to fasten your skin to the foam body. Stitch the along the top, bottom and armholes of the body and the bottom and mouth of the head (see photo of mouth stitched).

Create the underwear

For a first pass at the underwear - I really just eyed how to add the underwear. I didn't measure, I just draped the white fabric and pinned it to have the fabric follow the bottom edge of the egg shape (I show this in the underwear details step). Remove the pins that secure the white fabric to the body - then use the pins that remain as a guide to sewing the shape you need.

Step 3: Add Nose, Ears and Eyes

In this step, you'll create the nose and ears from pieces of foam and fabric and eyes from hot glue.

Step 4: Create the Cape and Attach It to the Head and Body

In this step, you'll cut a piece of felt (or fabric of your choice) to attach to the body to make the cape.

PAINTING/MATCHING THE CAPTAIN UNDERPANTS CAPE MARKINGS:

If, like me, you want the pattern/markings of the cape to be the same as Captain Underpants - follow the first set up subtask steps to create pattern. Otherwise, get a red fabric with black polka dots and move on to cutting the cape and attaching it to the body.

Materials you'll need for this step :

2 yards of red acrylic felt (least expensive) & red thread
2 jars of black fabric paint
1 small foam brush
1.5 yards of red velcro
10" Thin ribbon or 2 rubber bands

PAINTING THE FABRIC

Lay your fabric out onto a flat surface.
Pour the paint into a dish.
Dip the foam brush into the paint.
Dap the brush onto the fabric.
Vary the direction of each mark that you apply so that the markings look random.
Once you have covered the whole piece of felt in markings, let it dry overnight.

CUTTING OUT CAPE AND ATTACHING IT TO THE BODY:

First, position then pin the uncut felt fabric along the top of the body (neck) and down the back of the body piece. Then, figure out the length you need by having your model put on the body and mark the fabric where it falls just past the body. Once you mark your length - cut the rectangular piece.

Next, whipstich the edge of the felt to the top opening of the body (neck).

Even though we've attached the cape to the body, we still have to achieve the wrapped-around-the neck-look. So I decided to cut out and attach 2 triangular shaped pieces - one to each side of the cape. That additional triangle of material gets machines sewed along the side of the cape and hand stitched along the top of the body and bringing more fabric to be visible from the front of the body. The full costume photo above shows the costume without these triangular pieces added yet. And the 5th photo shows you that triangular piece added and how it extends the cape to the front.

CUTTING MORE CAPE FABRIC PIECE AND ATTACHING IT TO THE HEAD:

I took another piece of felt and pinned it to the neck to guesstimate how much I would need to wrap around the neck and knot. I wound up cutting about a piece that was 40" width and 4" in height. I created a duplicate piece and then machine stitched those together. Then, I lightly stitched that piece around the bottom of the head (leaving spaces of 3 inches between stitches to progress faster through the steps - I later filled in the spaces with more permanent whipstitches). You can see what it looked like in the full costume photo shown in this section.

To achieve the knotted look, I tied a red ribbon

ATTACH MORE FELT FABRIC TO HEAD TO COVER GAP BETWEEN HEAD AND BODY

As my son was putting on the costume as I progressed, I noticed that his natural movement would show a gap between the body and head. So in order to reduce that, I added more of the cape fabric that is attached to the neck (see last photo). To make sure that the gap is covered as he walks around, I added red velcro to this piece and all along the top edge of the cape. So now, the head pieces and body pieces are connected and the gap between these pieces is significantly reduced.

SCALLOPING THE BOTTOM EDGE OF THE CAPE:

You just have to eye this. And use pins to serve as a guide for getting the scallops right before you use a sharpie to mark the shape and before you cut. As you pin, pay attention to where the seams are in the cape. Make the seams where the longer point of the scallop is. After you are happy with your pinned scalloped shapes, then use a sharpie to make the inner edge of the shape. Once you are happy with that - cut just above the sharpie line so that you are left with sharpie free scalloped edge on your cape.

Step 5: Finishing Touches: the Underwear

Step 6: Finishing Touches: Add the Teeth

This should be one of your last steps. In order to achieve the look of the teeth and keep the visibilty for the weare of the costume - I'm using window vinyl typically used for signage on storefronts and car windows.

Materials you'll need:

VViViD One Way Perforated Window Vinyl Privacy Wrap Film Roll Decal Sheet DIY Easy to Use Air-Release Adhesive (1.5ft x 54") (this was the smallest piece I could get - try to get smaller)
Clear Film - you only need a 7x7 piece
white board erase marker -black
Sharpie - black
Adhesive spray

SUBTASKS

Cut a piece of perforated vinyl that will more than cover the mouth opening in the costume (leave at least 3 inches around)
Cut a matching piece of clear clear film
Peel away the perforated vinyl from it's paper
Lay the perforated vinyl onto the clear film
Lay the vinyl and film piece inside the head covering the mouth
Use adhesive spray sparingly and spot spray lightly to attach the vinyl film piece inside the head
Wait for it to dry
Once it's dry - lightly draw the teeth in - I used white erase board marker first to rough it in t
Once you're happy, then use the sharpie to work in solid lines for the teeth

Then you're done!!