Simply put it here assuming your game is installed on your C drive. It will overwrite the default blank "White" livery you see at the end of each car in game. You may want to make a backup of your original .dds file and save it in a safe place, if you need to go back to the blank "White" livery. So you'll have something like this:
porsche_991_gt3_rs_livery.dds and you'll put it in here:
C:\SteamLibrary\steamapps\common\Project CARS 2\Vehicles\Textures\CustomLiveries
then it will ask you if you want to overwrite the file. Say yes if you do and no if you don't.
Once you are in game, you will still see the "White" livery, but once you click on it and choose it, you'll see your new custom livery.
Hope this isn't too confusing and hope it helps