There are a couple of ways, the easiest being going to garage mode (driving into a garage on west coast or from the main menu). From garage mode, click decal editor, then click create new. Once done with that, you can save it, and name it. It will appear in the mods menu as an unpacked mod, then you can spawn the vehicle and apply it. The skin may be broken with updates or hotfixes, so be aware of that. The other way is a bit advanced, I don’t really know how to do it yet. Check this thread.
What car are you using? --- Post updated --- All base game cars should work, some mod cars work, automation ones don’t work.
gavril d-series --- Post updated --- ill try again --- Post updated --- it works now must have had a funny moment