I did this one for my daughter. Kinda rough, but I wanted it to look like a 3 year old's tricycle, not one of my motorcycle seats. If you have a good seat to start with, just remove the factory cover without destroying it and use it as a template. You'll have to disassemble it of course. Install it the same was it was attached before. If it's a metal seat pan, rivet it. If it's plastic or fiberglass, staple it. You can even cover the bottom in speaker cabinet carpet to finish up the bottom nicely. I'm starting on a seat today for a friend's son. He saved up his money and bought a way cool OLD schwinn sting ray and wants the seat done in leather. I try to get some pics throughout the process for you.
Cha-Chi