jordanl721Hey, you're question is pretty old, so you've probably found an answer by now somewhere, but I'll answer anyway.
After clicking "Join drop" you are taken to a page where you choose all the options you want. For this drop in particular you choose the poster stock (black chalk board, green, white etc) the car model, and an optional frame.
Also, clicking "Join drop" doesn't immediately commit you to the drop, you can still back out.