Apply your coupon either:
On the Shopping Cart page before proceeding to checkout:
Or on the Checkout page itself:
Clicking on the 'Have a coupon?' link will open a drop down where you can enter your coupon code.
After you click on Apply, you'll see the coupon value specified under the subtotal in the right hand column, the Total will have been adjusted accordingly
Note: coupons can only be used once unless we set them up otherwise.