You may redeem your gift certificate during checkout in the Payment Information Section. Then click "Add Gift Certificate" and input the information from your certificate. Enter your credit card information per usual. The amount of your gift certificate will then be applied to your order and the amount due adjusted.
If there is any balance left over it will be applied to the credit card.
If the amount of the purchase is less than the amount on the gift certificate, the remainder will remain in the system for a subsequent purchase.