I've done something like this with some success but I airbrushed the pattern I wanted because I didn't have any suitable software to create the images I wanted. I've only done roundels, fin flashes and name plates(on a ship, black letters, white background).
Assuming you have suitable software to create your design, you need "decal paper" and Micro-scale industries liquid decal film, which is a liquid. Both should be available from "Little cars.com". All the instructions come with the paper.
You need to be aware that the paper should be compatible with your printer,either inkjet or laser, so choose the right paper, apparently, inkjet works best.
You have a choice of clear or white paper, depends whether white is in you design or not.
I've got some clear paper and the decal film, I'll try to remember to bring it along to the next meeting so you can have a look.
Haddock.