Here are the instructions from RetailMeNot.com :
Thanks for taking the time to learn about the RetailMeNot service.
Did you know when buying items and services online there are often great discounts readily available?
During the payment process many online stores will have a field to enter a coupon or promotion code. Entering a special code will give you a discounted rate or free extras.
STEP 1: Make a note of the online store you are buying something from. For example:
Or even just:
STEP 2: Visit http://RetailMeNot.com
STEP 3: Enter the address from step 1 into the box and press the "Get Coupons" button
STEP 4: You should now be presented with any coupons available for that store. Make a note of them.
STEP 5: Click the 'use it' link next to the coupon and proceed to enter your coupon code when asked at checkout.
With any luck you should now have a great discount or bonus!