You'll click on the "Promotions" tab at the top of the page, and select "Manage Coupons" from the drop-down menu that appears.
Once you get to the coupon page, enter a "coupon code" and "coupon description" (this is how the coupon name will be presented to your customers).
Select the Minimum Purchase Requirements (you can select no purchase requirement if you want).
Choose the discount amount in either dollars or a percentage that you want to offer in the coupon.
Choose the beginning and end dates for the coupon offer to be valid by using the calendar tool to the right of the date boxes.
Mark the Status as Active and click "Save Coupon".
Customers will be able to enter the code you create into the "Promo Code box" near the bottom of the shopping cart page, and then click "Apply." The discount will then be applied to the total and the customer can proceed through Secure Checkout.