Taking the lead from this instructable (thank you very much!) I made my own version with tweaks to the original design. My design is probably a bit harder to get in and out of, but still a ton of fun for all.

Tools:
Google Sketchup w/Flattery Plugin
Printer
Scissors
Needles
Sewing Machine
Utility Knife
Rulers
Fabric Tape Measures
Black & Brown Sharpies
Dremel

Materials:
Brown roll-o-paper
2 Cheap camping mats
String
Gorilla glue
Yellow plastic mesh
1/4" Batting for queen size quilt
Yellow fleece & thread
Denim & orange thread
1/4 yard black fleece & thread
Velcro
Black pipe cleaners
3" rounded PVC pipe cap
4" flat PVC pipe cap
1 sheet plexiglass
1 Silver metal spray paint can
Nail polish remover
1" wide black nylon strap

Step 1: 3D Model of Head

Trying to follow the other instructable was very hard because I didn't know the scale of it, and how it related to the scale of mine. So after several failed template attempts, I decided to do start from scratch to get all the angles right. Using Google Sketchup I create the head in 3D. I have attached the file so you can adjust this yourself.

Ours body was 14" in diameter. I also wanted to put his view hole out above the eye to keep the scale spot on to an actual minion. This affected the curve of the head. I also didn't want the sections to go all the way to the top, but to have a disc in the center which should help with the build given the material.

Once I was happy with the shape I used the plugin called Flattery that takes the 3D model and allows you to flatten it so it can be printed (great for papercrafting!).

To get ready for flatting it, I displayed all the edges. Do this by either triple clicking using the selection tool, or right clicking with the selection tool, and going to "Select > Bounding Edges". Then right click and select "Soften/Smooth Edges" which brings up a dialog box. Make sure both Checkboxes are unchecked and the slider is all the way to the left.
With the flattery plugin, first you use the index tool, then the unfold tool. With the unfold tool select the have nothing selected then click each polygon to slowly unfold it. You really only need to do one section. As this can be easily repeated.
Finally printing to scale in Sketchup.
1. First change the camera's perspective. Do this via menu item "Camera > Parallel Projection"
2. You can then do top or front views "Camera > Standard Views > Top".
3. Center what you want to print with a minimum border around it (i.e. change the window size).
4. Then select "File > Document Setup", uncheck "Fit View to Page" Change the width and height close to what you are printing on, and set the print scale 1" and 1". It says it might not be measurable, but it was for me.
5. You are all set to print now.

head shape1.zip
Download

Step 2: Foam Base

First take the 14" diameter body and using pi (I always love it when I can use some math to solve things, plus helps teach kids and show how it is a needed skill!), I calculate the circumference which for me is about 44". Cut the two camping mats (can be found at walmart for ~$10 per) down to 44" long. Using that print off as a template, We cut the top edge of one of the mats ending up with a bunch of finger like things. You could also do a bottom rounded edge using the same template, but I didn't think it was necessary.

We then attached the two pieces together since the mats are only 20" high. We did this with a scrap piece of mat material as a backer and Gorilla glued them together. Then we used some hot pink nylon string as a thread and a large needle to stitch the pieces to the scrap backer piece. I thought the backer piece was needed for added strength.

Next we made a cylinder out of the mat by bringing the two edges together. Again using a backer piece and string. In the pictures you will see that we only went down a little ways. If I were doing this over, I would have done the whole way down because of the way the costume went on. Original thought was to have that slit in the back as the way in and out of the costume. But once we did the overalls, this was quickly changed. Anyways, the take away is make a cylinder, and don't do what we did.

After the cylinder was made we put together the dome at the top. At first we were sewing each piece all the way down. After realizing this was a bad way to do it because the dome would not be formed properly just going from neighbor to neighbor section (not including a waste of time and string), we would put two or three neighboring sections together at the top, and then attach them to the top disc. Then move to another section on the other side. This allowed for better formation of the dome. With the cylinder and dome done we cut holes for arms and the spot where my son was going to look out of.

To keep the scale correct my son would stick his arms out of the costume at the elbow (i.e. keeping the upper part of his arm at his side). This happened to put the arm holes right at the joint of the two mats which sucked, but wasn't a huge deal breaker. Hindsight, I would have figured out the over all height needed before and shortened the cylinder first before doing the dome. That way the arm holes would have missed the joint.

The viewport was really just a guess and ended up going a little two low. No biggy since we could cover it up with the minions eye later. We had to sew all around the viewport's dome sections since it was right there.

Last we cut the cylinder down in height, so the end of the costume was right above my sons knees.

Step 3: Skin

Skinning involves three components: Viewport mesh, batting, and fleece.

Viewport mesh is the same color as the fleece and works very nicely to hide the wearer. Just stitch it with the nylon string to the foam.
Batting was the thinnest I could find, then we doubled up the layers. I didn't know what we would need, but the double layers made it nice and soft, plus blocked the blue out from behind the fleece. I just used white thread and stitched it to the foam. The top of the dome got a larger circle then what was there on the foam.
The yellow fleece on the top followed the same as the batting. This way there wasn't too much of an issue with the wrinkling.
1. First take a measurement of the circumference and make the cylinder. Fleece stretches pretty good so you can make it tight.
2. Next cut out the top cap, and sew that on. This is where my limited sewing skills showed up. I think I was stretching the fabric, or didn't quite get the right size because there was a wrinkle at the front. It was a good thing it was at the front though, since the eye would be able to hide it, along with the viewport.
3. Slide on the fleece. You can use spray mount to attach the bottom edge of the fabric to the inside of the cylinder.
4. Next cut out the viewport and arm holes. Then stitch them to the side.
5. Make some sleeves, measuring from just about the elbow since that is where it will be coming up to. I stitched this both on the inside and outside of the costume for added strength.
6. You can see on my shots where I put in velcro and the fleece went inside the cylinder. Again DON'T DO THIS. it is completely not worth it!

Step 4: Overalls

I want to start by saying these are real overalls, made with denim and proper thread. They pretty much had to be for the costume to go on and off in a usable manor. With that said, they were a pain in the butt to make, especially since this was my first sewing project on my own. There are lots of mistakes, but you know what, it still looks great to me :) So if you aren't the best, just go with it, and be happy with the results! Anyways, enough peptalk, back to the build.

I started with a template that I though would work. But the only part of the template that I used was the parts that were touching the cylinder. From the cylinder I just came straight down to the length I needed (so it looked like a denim dress with an overall flap on it). The curve to go from a 48" waist to a youths knee's were just too much for my brain to calculate. The pinning method worked well for me :) Just figuring stuff out on the fly.

One of my favorite parts of the build was the stand that I threw together. It is 4 hockey sticks zip-tied together. Made things so much easier!

The pocket with the Gru logo on it was just a black sharpie drawn on it. And then we added buttons and straps to complete the look. Also... button holes are a pain (at least with my limited skill/knowledge/tools).

Step 5: Eye, Goggle, Glove, Hair, and Mouth to Finish It Off

The last bits were the fun to make.

Eye:
My son only wanted one eye, which made me happy since I didn't have to figure out angles for the eyeballs and goggles sitting on a curved surface. Also helped hide the sewing mistakes in the yellow fleece. However it cut down on his visibility.

Starting with the rounded plastic PVC cap, I used a coping saw to cut just the rounded end off. Then used nail polish remover to get ride of the bar code that was there. Using a black fat permanent marker as the base I made a large round circle for the whole eye. Then I went back over top of that with a brown sharpie to give it some nice depth. Finally using a black sharpie for the pupil. This got glued to the fabric with Gorilla glue.

Goggle:
Using the 4" PVC cap, I dremeled out the cap part leaving a little lip for the plexiglass to sit in. I cut two slits in the cap for the nylon strap to fit though. Then panted it a metal color and sewed the straps to the side. Should have probably included something in there so we could tighten it over time. I cut a circle out of plexiglass using the dremel again (and the goggle as the template). It fit nicely... however I didn't look closely enough at the plexiglass to notice it was frosted :(

Gloves:
Minions only have three fingers, so the gloves were made to match. See picture... but basically trace hand and leave enough room to sew and put on.

Hair:
I cut 4 small holes in the top and pushed through the pipe cleaners, looped them inside the costume and pushed it back up through. Then twisted them together. 4 pipe cleaners pushed though two holes comes out to 8 strands of hair.

Mouth:
The mouth is what makes this costume I think. My son wanted the mouth where the minion says "WHAAAA". Cut the shape of the mouth out of black fleece. Then the teeth out of white, and tongue out of red fleece. Using a black sharpie I colored the edges of teeth and tongue. Then added shading using the sharpie as well. The used spray mount to attach it to the costume.

Step 6: Have Fun

Here are some extra pics of my boys having fun at the mall and parade :)

It was a hot costume at the mall and he needed a break, but it was great at the parade where it was a nice cool New England Halloween!