Once you add all the products you wish to buy to the shopping cart, you will see the 'Coupons' section on the same page.
Select the coupon code.
Once the coupon is applied, the discount will be shown on the bottom of the same page under 'Price Details.' You can then proceed to checkout and pay the remaining amount using any other form of payment.
Note: Only one coupon code can be used on a single order. If you cancel an order after applying a coupon code, the code may not be usable for a later purchase, depending on the coupon's conditions.