Hey, so Mrs. Minion here is competing in minion costume contest. The costume took about a week (mostly because of the paper maché dry time) and cost about $50 (less if you already own an exercise ball).
Supplies (there's lots, but hey, it's an awesome costume)
Exercise Ball (slightly wider than yourself)
Paper Maché - Mix 1 part water with 1 part flour an a tsp of salt.
Construction Adhesive (Either Liquid Nails or Loctite will work)
2 empty/new quart size paint cans (sold at Home Depot in the paint section)
Yellow Spray Paint
Craft Foam Sheets, 1 black, 1 white
small nuts and bolts
Empty Paint Cans - They sell them right across from the spray paint at my Home Depot. You need 2.
Blow up your exercise ball and measure the diameter. Now roll the polywall panel into a tube until it's the same diameter as your ball.
(1 part flour, 1 part water, a little salt. Exercise balls have a seam around the middle which really helps in getting a perfect dome. Just paper maché past the seam a little.
Attach the paper maché dome to the polywall tube you made in step 1. Let the ledge dry.
I trimmed my dome with scissors to the seam line the ball made in the paper maché. If it doesn't fit, keep trimming the dome little by little until it slides in (or you force it).
Cut arm holes and a mouth.
Paint the whole thing bright yellow.
Make the goggles. You can't cut the bottoms straight off.
Make eye balls! Cut a cheap small rubber/plastic play ball in half. Flip the ball inside out. Slide it in the goggle from the side you cut. Attach with duck tape. Be careful... the paint cans are sharp!
Attach the goggles with eyeballs to the minion (now that it has eyes, we'll call it a minion). I taped the paint cans together, ran a bead of construction adhesive around the rims of the paint cans, then taped them to the minion on the sides. Center the eyes right over the seam between the dome and the tube. If you put duck tape on the sides of the cans, the goggle strap will hide it.
Cut goggle strap, teeth, and hair from craft foam sheets. Hot glue in place.
Hot glue some blue fabric to the minion so he's not naked. Your minion transformation is complete.