Step 1: Getting Started
A 3D design program. I used Autodesk 3Ds Max (because I'm a student :), There are free alternatives such as Google sketchup for those who aren't students. I don't know how to use that software but megaduty has posted many sketchup instructables and was nice enough to help me start using the program.
Thats all you need to design it.
Helpful items, A bike, A Ruler.