I have a TON of 9"x13" chipboard (the pressed together cardboard stuff, not the wood product) left over from a previous business that I use for my patterns. I typically draw them out on my PC in one of a couple programs - Inkscape is great and free - and then print them out, double sticky them to the chipboard sheets and cut them out there. Once I have the patterns done, I use a combination of chipboard to simulate thicker leathers and some hideous upholstery fabric to make test runs from my patterns. It's not perfect, but you can punch and sew it well enough to at least make sure everything will fit together right when you're done without wasting any leather. The chipboard takes some getting used to so that the folds and bends somewhat mimic the way leather moves.