First find where AC is installed, than go here:
Steam\steamapps\common\assettocorsa\content\cars
If you have like a skin for the Abarth500, go in the Abarth500 folder and you see a folder called skins, you need to place the downloaded skins in there, make sure they have their own folder!
And when you want to use a skin ingame, you need to click on the color tab at the car you want to use, that will bring up a side menu showing what colors aka skins you can pick!