[Help] Livery making

Ive been trying to make a livery for this car https://www.gta5-mods.com/vehicles/subaru-impreza-wrx-sti-2004-add-on-tuning is was a template but doesn’t have any livery slots, so when i make the livery and put it on, it can only have that livery and cant take it off. it also looks like there’s a layer of paint on top of it unless the boby is painted white

does anyone know how to make a livery slot for it so you can do /livery and change it?