Follow these steps to redeem your Steam Gift Card:
- Sign in to your Steam account in your browser
- Go to the Redeem a Steam Wallet Code page
- Enter the Steam Gift Card which you can find under My Withdrawals by clicking on "Show Code"
- The funds are added to your Steam account and will be ready for use to buy skins, games, and anything else from Steam!
Note: You can find the official instructions here.