Custom car livery skin help

I am currently making a police livery skin and I can do that on cars that have templates (UV-Mapped) but as soon as I put it on a car that was made from one of my modlers it goes all weird and completely different to how it’s meant to be.

So do you know how I can make a livery skin on a custom car?

If none of this makes sense or need clarification please let me know!