The Nipple Erector Set has everything you need to get your nipples pumping with pleasure! To begin, choose the pump with the smaller opening and place it over your nipple. Squeeze the pump to get to a desired perky state then slide the rubber ring around the nipple to keep it erect. Repeat the process on the other nipple. To make the nipples even larger, use the pump with the wider opening. The 4 o-rings come in varying sizes. The thickness of all rings is .75in. The widths are: XL = .6″ L = .525″ M = .440″ S = .375″