Once you have your selected items in cart, "proceed to checkout".
There are 4 stages of the checkout process:
Address, Shipping, Billing, & Order Review.
After you complete the Address and Shipping portions of checkout, you'll see the orange "Add Gift Certificate" button on the Billing
page (see screenshot below for example.) Simply click this button and enter your Gift Certificate Id in the field provided, then click the "Add Gift Certificate" button and your Gift Certificate will be applied to your order. You will see this reflected in your order total.