It really depends on the car, and on what kind of skin you are trying to make, there is no universal template for every skin for every car
I got a skin made for the Gen 6, but i cannot get it to load up into the game. I copied the materials and jbeam files, and made the skin on Paint.net as a .DDS file since that's what the main mod uses for their skins. I just cannot get it to import in, would anyone want to look at the mod file i made to see where I'm going wrong? I'm really not sure at this point lol