Log in to your League of Legends account and enter the game. On the top right side of your client you'll find a "Store" button. Click on it, then click on "Purchase RP" button.
Choose the country you're from, then select the paysafecard payment method on the left side and the amount of RP which corresponds to the reward you've ordered from Gamehag.
After an other window appears, enter the code you've received from us. After reading the Terms of paysafecard, click on the "Pay" button.