Measure the trim as best you can leaving at least a half inch excess on each side of what you want to wrap, ideally more like an inch just in case. You can do this on the car in most cases, just make sure to account for any curvature or irregularities that you might run in to. Once you have measured each piece you want to wrap, you can figure out how much material you need to order. It typically comes in a 24" roll, so you'll need to figure out how many feet you need from that 24" roll. You can find the stuff all over the internet, including eBay.