Add all your items to the cart, then click the "Checkout" button. You will see a "Coupon" line right above the shipping options. When you tap it will prompt you to enter the code. Click enter to apply your code. As long as the code is valid it will be applied to your order and show up in your order summary.
If your code isn't working it may be expired or you may have the wrong item for that coupon. Please email us and include a screenshot of your cart if possible, so we can help make it work.