The trick to executing a sucessful kneetap is to have your opponent's weight on one leg, and then to bring his body across it.
Assuming you are using your right arm to underhook:
I would start by bringing my opponent's left leg forward. When his weight transfers to that leg, block behind his left knee with your left hand. Drive the underhook up and across his body- the motion is similar to throwing a baseball.
If you really want to see some slick knee taps, look for videos of Cuban wrestlers Quintana and Garzon online. A perfect example is at 1:30 and at 4:54 of this bout.