If you have a valid coupon code, you can enter it in the coupon code box on the checkout page to receive your discount. The coupon code box is revealed by clicking on the words  "Apply Coupon Code" located on the checkout page below the Order Summary.  

You can then enter the coupon code and press the Apply Discount button. 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 Summary.