To use a coupon you must first login or create an account. After logging into your account, click on coupons as shown in the image:
Clicking "Coupons" will take you to the Redeem coupon page.
Enter your coupon code in the required field as it shown and click submit.
Once you use the coupon you will see a success message in the green box confirming the process.
and this means that the coupon value is added to your account.