You have to go to modworkshop for that theres tutorials on youtube for downloading mods and what you need so just search some up and for any skin at all if its a folder pic with a zipper than right click on it and click extract if its a blank white page download 7zip and right click on it once its downloaded hover your cursor over the 7zip thing and then it should say extract to (almost the same name as the file if not the same) and there you go. Its going to appear as a new section on your custom colors saying mods or it will appear in the actual skin bar if your game crashes its in the wrong its supposed to go into the mod_overrides folder the tutorials will tell you more.

If you can make your own workshop skins, there a mod called usable workshop to test them.