Rub them gently until you begin to feel them harden. Once you reach this step you must the take a cold hand and flick them. After this your nipples should be stimulated. ;)
the way one can enjoy stimulation is by pulling ur nipple in and out squeezing it pinching it ,but if u staring pulling at it ,it can grow big and can secreate some fluiud and it will often get erect...