Once you have your selected items in cart, "proceed to checkout".
There are 4 stages of the checkout process:
Ship to, Deliver by, Pay with, & Order Review.
After you complete the Ship to and Deliver by portions of checkout, you'll see the orange "Add Gift Certificate" button on the Pay With
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.