Go to http://www.steampowered.com/wallet and sign in if you aren't already. You will need to create a Steam account if you do not already have one.
Once logged into Steam, please enter your unique Wallet code that you received into the “Steam Wallet Code” field.
Click "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 next to [Your username]'s Account.
When in the “Review + Purchase” step during checkout click “My Steam Wallet” as a payment method. The cost of the item you purchase and the associated tax will be deducted from your wallet funds.