Any plank of wood or plywood should do, for my board i bought a shelf at goodwill for about 50 cents, its about a foot and a half long and its approximately the same width as my trucks. Now you don't have to use wood, anything that can support your weight can be used get creative!
After you acquire your wood draw out your board shape (or leave it as is) then draw a center line. A center line is a straight line a that is in the middle of you board, it runs length ways and with it you can line your trucks up to make sure they are straight. Now don't drive yourself nuts trying to line the trucks up, they can be a little off.