Open Steam, login, and click the tab with your name in the top right of your window. Select ‘View my Wallet’ in the list. On the right center of the screen select ’Redeem a steam gift card or wallet code’. Enter your code on the next screen, then press Continue. If it doesn’t work right away, wait a little while and try again. Sometimes it takes a little time for the card to activate on Steam. Code is revealed by gently scratching away the solver portion on the back of your gift card.