If you have a valid coupon code, you can enter it in the coupon code box on the checkout page to receive your discount.
For a step by step guide on how and where to use the coupon code, click here.
If the code has worked then you will see the words "Coupon accepted" appear next to the box. You will then see the discount added to your Order Contents at the bottom of the checkout page, and you can proceed to enter your payment details.
If the code doesn't work, check to make sure you've typed it correctly, it hasn't expired and its valid for your purchase. For example, some codes only apply to particular products, particular shipping locations or if you have received a set spend limit.