Adding custom vehicle skins to your server can be quite tricky if you don’t know what you’re doing. However, I suggest going to this tutorial and following all the steps as it will help you.
https://forum.cfx.re/t/how-to-stream-vehicles-models-from-server-to-client/274
Furthermore, you are going to need this link to download custom skins off of
http://www.lcpdfr.com/downloads/gta5mods/vehiclemodels/
Keep in mind, after you’ve followed the tutorial it’s fairly easy to put the skins on to your server.
Hope this helps!