On the checkout page, just below the Total is a box. Enter your coupon code and click the "Apply" button.
Make sure you see the amount change so that you know it has worked.
We also have a page that explains here https://www.bluecoffeebox.com/redeem
Your coupon code must be valid and must be redeemable against your selected product. Some coupon sites feature old, expired coupons as a way to make people come to their websites.