To create or edit a coupon/discount code follow these steps
Log in and go to Dashboard
Commerce
Discounts
Add a Discount or Pick the Discount you wish to edit
Fill in all fields with an *
Formula can either be a dollar amount or percentage
Check the Enable box to turn coupon on
Start Date is when the coupon can first be used
End Date is when that coupon can no longer be used
Max uses per coupon is how many times that coupon can be used total - If set to 1 coupon code can only be used 1 time
Max uses per user is how many times an account can use that coupon code - this should be 1 so users can't use the discount more than 1 time.
Enter Discount Code(s)
Click Add Code
All codes assigned to this discount will display in Current Codes window
Save