There are many websites out there to download skins these are the fastest and easiest ways to download them. Click on the skin you want and click download, once downloaded click open at the bottom of your screen, before you do so have osu! opened, and then it will automatically put the skin for you in-game. If that does not work, click save as, and then go to your file explorer and go to quick access or downloads folder and drag and drop it into osu. To do so open osu! and have file explore not in full screen but in a minimized state and then drag and drop your skin into osu! it will say .osk for the file address