Identify the proper time to throw a knee kick. If you are grappling with an opponent, look for an opening where you can knee-kick him in the ribs or side of the thigh. If an opponent is trying to tackle you around the waist or take you down by grabbing your legs, introduce his face to your knee. If the opponent is standing in front of you and there is not enough room to throw a fully extended kick, grab him and try a knee.
Launch a knee kick from a front stance. This is longer than the typical fighting stance. Your plant leg should start off bent. Your kicking leg should be about two steps behind. Your balance must be solid. No wobbling. If you are in a fighting situation, slide your kicking leg back or take a long step toward your opponent before launching your knee.
Reach forward with both hands and grab your opponent. The knee kick works best when you pull your opponent toward you as you thrust your knee into him. It also works best when thrown across your body. That way the full range of your hip is used. For example, if you are attacking a standing opponent with your right knee, grab their right shoulder (on your left as you look at him).
Launch your knee upward and forward as you pull your opponent toward you. Imagine that your opponent is a stick that you are trying to break over your knee. The impact of the knee and the pull combined is far more than each on their own. Build as much speed in your knee as possible. The further back your kicking leg is, the more room you have to build speed.
Throw it again. The wonderful thing about a knee kick is that once you are done, you are in position to throw another one. As long as your opponent is still there, you can keep pounding them with knee kicks. Just slide your kicking leg back and go again.