Proceed to the payment section of placing your order. Above the credit card section, click on Have a Gift Card or Promo Code.
This will open additional payment options for your order. Enter in 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 in another method of payment to cover the remaining balance.