In order to redeem a code for League of Legends items:
Go to https://signup.leagueoflegends.com/en/signup/index (skip this step if you are already a registered user).
Register a new account by filling out required fields (skip this step if you are already a registered user). Important! Pay attention to the region you select, this may have influence on the codes which are region-bound, on your your ping times, as well as on being able to play with your friends.
Download League of Legends game client and log in (the client may have to update the game). (Skip this step if you have already installed the game client).
In the client, enter the store in the top middle (next to the "PLAY" button).
In the shop, scroll down, on the bottom left side there is "Codes" tab, click it.
Enter your code and click "Submit".
WARNING! This product can only be used on EU servers of League of Legends.