I just used some hand drawn sketches scanned in, then just began creating pathes around each portion of the image. Then with the selection still active, I use the airbrush tool and begin shaping each piece. The end result for things like these is usually a file w/100+ layers that I can then put together kind of like a model. For instance I created different wheels and rims for the car that I can change or whatever. I just couldn't decide while I was making it!