How to redeem steam gift card?

Faq, Steam

A: Step 1: Visit the Steam Website or open the Steam application.

    Step 2: Click on your profile name at the top right-hand corner.

    Step 3: Click on “View My Wallet”

    Step 4: Click on “Redeem a Steam Gift Card or Wallet Code”

    Step 5: Input your gift card code in the space provided. After you are done, click continue.

See also why your steam gift card is saying invalid