Start off with a good template to help lay it out (we use pro vehicle outlines and they are very nice), then just design the layout as a large rectangle over the template, make sure to add plenty of bleed (depending on how curvy the vehicle is it could even be like 5" extra on each side), and the most important thing is hand measure the vehicle to know exactly where text and things need to lie to avoid being cutoff in wheel wells or in door handles etc. don't trust the templates for the final print.
Pretty easy, just check and double check those measurements and you should be fine.