Step 1: Design
The other considerations after that are the wheel size and the distance between them. With that template in place, it's car time! Here you can go nuts. You can make the car look realistic or abstract. It can have a front and back or be symmetrical. Here I'm going for abstract and symmetrical, but you can make your car any way you want.
The image here shows the layers of wood all stacked up over the wheels as well as all laid out for a cutting file.