You can redeem codes directly from the Steam desktop app., Make sure you sign in with the same account you want to apply the wallet funds to.

If you don't have a Steam account, you can follow the prompts on the login screen to create one for free.
 A menu with a couple options will appear., This will open your account details page in the Steam main window., This will open the page for adding funds to your Steam wallet., This will allow you to enter the code to add funds., Carefully enter the code to redeem it to your account., If you've never made a purchase on Steam before or have never added any funds to your wallet, you'll be prompted for your address so that the funds can be converted to the proper currency. The address doesn't have to be real if there is no payment method like a credit card associated with it., Once you've entered the code, you'll be shown how much will be added to your account. Confirm that this is the correct amount.

If you are entering a code from another region, the total will be converted to your local currency using the day's exchange rate.
 When you're making a purchase, select "Steam Wallet" as the payment method to pay with your wallet balance. If the balance is insufficient, you'll need a secondary payment method to pay for the rest.
Start the Steam desktop application.
Sign in if you aren't already signed in.
Click your account name in the upper-right corner.
Select "Account Details.",
Click the "+ Add funds to your Steam Wallet" link.
Click "Redeem a Steam Gift Card or Wallet Code.",
Type in the code that you want to redeem.
Enter your address if prompted.
Confirm the amount that will be added.
Apply the wallet balance during checkout.
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