1) Click on the Coupons button in your account's main navigation
2) Click on the Create Coupon button
3) Make your choices regarding the coupon and then click on the Save button. (Keep in mind that coupon codes are case sensitive)
4) Now, if you give this coupon code to a payer and if you've enabled coupon codes to be used on a payment form, the payer will be able to apply this coupon code during checkout.