First, go to the "My Gift Cards" section under "My Account". Then input the entire Gift Card code, including the dashes, and hit Apply. Please make sure no spaces are copied over if you are copying directly from the Gift Card email.
Once the code has been applied, you will see "Gift Credits" change from "$0.00" to the amount of the Gift Cards' value:
Once you have redeemed your Gift Card amount, you will proceed to the checkout process! For step 1 (Shipping), you will enter the name and shipping address you want this order shipped to. Then, for step 2 (Billing) you will want to click "Pay with Gift Card":
From here, you can then click the check box to "Apply Available Balance to my order":
Note: If the Gift Card amount covers the cost of the order, only a billing address will be required for the order to process/checkout completely. If the Gift Card amount does not cover the cost of the order, you will be prompted to add a payment method. You can click "+Add New Credit Card", select a credit card already attached to your account, or choose Paypal.