It is super easy. You go to Ebates, search for which site/store you want and then go to the site/store from there. Ebates tracks the page and once you make your purchase will get a certain percentage back. Every store/site has different percentages and some even offer other coupon codes for free shipping. The best way I have heard it explained is that it is like going to the mall. The mall being Ebates, you have to go into the mall in order to get to the other stores.
Hope that helps.