How to Redeem:
Once logged in, enter your Steam Wallet code here: https://store.steampowered.com/account/redeemwalletcode.
Select 'Continue' to redeem the credit onto your account. Note: If you have never used Steam Wallet credit on your account, you may be prompted to enter your current address to determine the correct currency.
The value of the Steam Wallet code will be added to your account's Steam Wallet balance. The current available balance will be listed in the upper right-hand corner below your profile name.
During checkout, under 'Review + Purchase' select 'My Steam Wallet' as a payment method. Your gift card balance will be applied to your order total.
How to check your balance:
Sign in to your Steam account.
Click on your email on the upper right-hand corner and on the drop-down menu select, 'Account details'.
Your current gift card account balance will be displayed under store & purchase history.
Abridged terms and conditions:
Steam Gift Cards work just like gift certificates, which can be redeemed on Steam for the purchase of games, software, hardware, and any other item you can purchase on Steam.
If you have never had a Steam Wallet balance on your Steam account, the funds will be added in the currency of the Steam store. If you have an existing Steam Wallet (regardless of the current balance), the funds will be added in your Steam Wallet's current currency.
Help Center: https://help.steampowered.com/en/