Vehicle wraps are a "black art" so you won't find many people handing out this information for free (at least until my site launches) but ill try a quick run down for ya. Background: over 200+ vehicle wraps designed, 15+ years in the industry. Also designed the Reddit Internet Bus. This advice is for a full wrap, on 3M vinyl. I highly advise against Avery and other materials.
EDIT:***God reading through this thread their is some awful advice, please don't use ANYTHING other then Pro-vehicle outlines based in Canada. They are the industry leaders and guarantee all of their template to within a 1/4in. I have no idea who or what any of these other companies people are referencing. I do know though, stay away from Bad Wraps.
Im a firm believer that every wrap should be done in Photoshop and every shop I have worked at here in AZ does it the same way (ive only met one other designer that did everything in Illy and that's cuz he was old school race car designer) Its basically scaling your pro vehicle outlines to 166.66% in illustrator, bring that in to a new PS document at 1200 dpi. Paste it as a smart object.
Image size> bring your dpi to 100, this now give you a 1:1 ratio (1 foot to 1 inch) which is important when giving bleeds. Now you add 5in to the front, top and bottom for bleed. Depending on the vehicle I will go all the way up to 15" of bleed on the back (popular with chevy vans where the actual "back" of the template starts at the doors, so you have to compensate with grips of bleed for the rear corner panels) normally I give 10in.
I now select my layer with the outline on it, I put a 3-5 magenta stroke on this in the layer style options and keep it on the top layer so I have a quideline.
Now comes the fun part. DESIGN AWAY!!! I always start with the most difficult side (busses would be the passenger side with all the doors, same with recreational vehicles, always start on the passenger side first) it will have the most crevices and shitty lines because of all the doors/windows depending on vehicle.
Once you have one side done you go through the first steps I gave you for bringing your drivers side in to photoshop. Set it up with the same bleeds etc. Now go back to your first design, the completed one. Select ALL of your layers in the pallette, go to edit>transform>flip horizontal.
Now you have a mirrored image, go to the options panel in your layer pallette and select "duplicate layer" and push all those layers to your drivers side. Now, individually select each layer in your layer palette and flip them back, edti>transform>flip horizontal. Now your mirrored EXACTLY like the other side.
Bring in your back, add 5-10in of bleed on the template in photoshop. Make sure you add your magenta stroke to your template. You can "duplicate" elements from your side wraps into your rear now and scale those to fit.
To finalize this for print? Simple. Turn off your guideline layer, flatten the image and save it as a .tiff. Your done! The RIP system will determine your overlaps for the installers, you have given bleeds so you are good to go!
If you have any questions, give me a holler at firstname.lastname@example.org