After doing mock-ups on the car using wallpaper lining paper, much better than messing about on photoshop, I did the artwork myself in Adobe Illustrator, then got a company we use at work to source some cream/black vinyl and cut them out for me. I applied them using a spray bottle full of water and a few drops of baby shampoo, it helps the vinyl slide about until they're in the exact position. Any car/van livery company should be able to do them for you or help you. The important thing is to make sure the car is super clean and you don't get any grit/dirt under the vinyl as even the smallest amount shows up.