Mobile:

1. At the checkout page (not the cart page), click on "Have a discount code? Click here."

2. When your "Order Summary" appears, enter your code in the discount code field and validate.
Discount code mobile view
Discount code mobile apply

Desktop:

 At the checkout page (not the cart page), enter your code to the discount code field located on the right hand side of the page under your order summury. (see picture below)
desktop-discountcode