Follow these steps to redeem your Amazon Gift Card:

  1. Sign in to your Amazon account in your browser
  2. Click on your name at the top left to go to the "My Account" page
  3. Click on "Apply a Gift Card to Your Account"
  4. Enter the Amazon Gift Card which you can find under My Withdrawals by clicking on "Show Code"
  5. The balance has now been added to your Amazon account

Note: You can find the official instructions here.