During checkout, or in the cart, there is a box below the shipping calculator where you can enter a valid coupon code.
If you select “Apply” after entering the code, the discount will be added to any applicable items in your cart.
Please note, coupons are applied to individual items or the order subtotal and will not provide any discount on shipping or tax. You can view more information about this, as well as other terms and conditions related to coupons here.