For those of you that don't know a onesie is a t-shirt with snaps in the crotch. You can pick the up everywhere for babies but for an adult you need to order form a specialty dealer. They normally start at $25 plus shipping and go up from there.
Step 1: Materials
For this project you will need.
1. A T-Shirt a few sizes too big. I wear a size large and used a 2XL
2. A t-shirt that fits for a pattern
4. Sewing equipment. I sewed mine by hand but a machine would be faster.
5. Pins. Lots of them.
6. A marker or chalk for lay out.
7. A tape measure
8. Snaps. I used sew on snaps but Velcro could also be used. (9mm shown in picture have since been replaced with stronger 21mm)