Log in to your Forge of Empires account and enter the game. On the top taskbar, on the right side of the screen, you'll find a counter of the diamonds you own. Click on the plus icon next to it.
Choose the pack of Diamonds which corresponds to the amount from the reward you've selected on Gamehag and select paysafecard as the payment method.
After an other window appears, enter the code you've received from us. After reading the Terms of paysafecard, click on the "Pay" button.