At checkout enter your discount code in the discount code box (if on desktop it is in the right column, on mobile scroll below product in cart) to apply your discount on your order.

If you enter a discount code during checkout that doesn't exist or isn't currently active, then you will see the following error message: Unable to find a valid discount matching the code entered. 

If you enter a discount code during checkout that does exist but doesn't apply to the products in your cart, then you will see the following disclaimer: Your cart does not meet the requirements for the _DISCOUNT NAME_ discount code.