Have you seen the guys that are good at it? They pinstripe everything. They even pinstripe their toasters. I don't think it matters what you practice on.
The pinstriping book I have suggests a piece of glass with some college rule paper behind it for a guide.
get a peice of glass, on one side mark out a grid in grease pencil, then paint over it all with a color of your choice, on the other side, practice, erase with thinner or whatever...use over and over.....when you are more confident, stripe everything that will sit still long enough....