Prime's profile levelling system is what gives you free skins. After you gain 5000 experience points to reach the next level, CS:GO gives you a drop at the end of the match. It can be a skin or a graffiti. Graffities are usually very cheap, but some skins can significantly boost your inventory cost.
To obtain this free skin and champion simply visit the official League of Legends Facebook page for your server region and like the page. Once the page has been liked follow the instructions on screen to link your account and claim your free champion and skin.
