When you purchase a gift card, you will receive an email that contains a unique gift card code that you can use at checkout to redeem its value. (See "How to buy a gift card", if you need more details.)
Once you've added all your products to your "shopping cart", just click or tap "Check Out", then on the next page, you can enter your gift card code and click "Apply".
If your purchase amount is more than your gift card amount, you can pay the rest via another form of payment (PayPal, credit card, etc.). If your gift card amount is more than the purchase amount, you can use the same gift card code to apply the remaining balance to your next order.
[If you need more detailed instructions on completing the check-out process, click here.]