To apply your promo code, type or copy and paste the code into the 'ADD VOUCHER OR GIFT CARD' box, through checkout.
Once you have entered the code, click onto 'APPLY' and it will add the code to your order. When the promo code has been applied, the value of the promo will appear and the total of your order will change. A message will also appear on the checkout page indicating that your coupon has been successfully added.
The only way to enter and take advantage of a promo code is when you are checking out as it can't be applied later.