Proceed to the Payment Section of the checkout process. Above the credit card section, click on Have a Gift Card or Promo Code.
This will open additional payment options for your order. Enter the gift card number and PIN into the Gift/Refund Code field and click apply.
Once this is completed, if there isn't a remaining balance you can complete your order. If there is a balance, you will be prompted to enter another method of payment to cover the remaining balance.