If you have received a gift certificate by email or in the post, you can redeem this at the checkout. 

Physical cards have the code on the back, printed underneath a barcode. Gift certificates sent by email include the code in the email. 

Enter your code in the box labelled "Gift Card of Voucher Code" as shown below and click "Add Voucher", your new discounted total will be shown in the Order Summary on the right and at the bottom of the page and you can proceed to purchase your items.

If the credit is not all used, it will remain in the account for future orders. You can view the remaining credit and the expiration date in My Account under the tab Gift Vouchers. Because the credit remains on the account, the code cannot be entered at checkout again. Instead, the credit will automatically show as a deduction from the total (if you are logged into the same account).