1. Make sure you use a stretch stitch! It allows the seams to stretch without the thread breaking, and it isn't distracting as a top stitch if needed. Here's my favorite stitch to use:
3. Invest in a walking foot. It moves the top and bottom pieces of fabric together, which reduces puckering in stretchy fabrics and vinyl. It also is fine to use on non-stretch fabrics.
4. For a pattern, I use a zentai suit that I took apart. If you use this method, use a zentai suit that's a touch larger than needed in case the fabric you use isn't quite as stretchy.
5. If you want to use a mesh overlay, all you have to do is cut the mesh out of the same pattern. Make sure it's 4-way stretch, or you may not be able to put on your suit! If you want, you can sew the edges of the mesh and spandex together before putting the suit together, but I find it's not usually necessary.
6.Before adding anything on top of your base suit, make sure you fit it first.
7. If you want raised/padded portions such as on the Old Snake costume, all you need to do is cut the same size of a thin piece of foam and fabric to cover it with, and just top stitch the fabric around the foam onto the suit. Make sure the fabric is pulled tight! If you do the reverse of this (foam and fabric on the inside of the suit) you can make a really clean looking muscle suit. I use the thinnest green foam found at JoAnn's.