Since this question has been asked multiple times, and a picture is worth a thousand words, I thought I'd post two ways to get on top of your opponent when starting from knees.
Firstly, you need to have good base and posture. So keep both of your knees on the floor, and your butt on your heels. Keep your arms in close to your body (think Tyrannasaurus Rex). A lot of people reach out like zombies and try to throw their opponent which opens them up to double and single leg attacks from the knees, duck-unders and snap-downs. It also means that if you do this against someone stronger then you'll lose. Now get in close so your forehead is close to theirs.
Use your right hand to quickly push their head to the side.
Drive your head into the left side of their neck.
Re-align your body so your spine is straight (so you're now off on maybe a 15 degree angle). Their spine should be bent and yours straight. Your right hand grabs their pants at the knee, your left hand grabs their right elbow and you drive into them putting them on their back.
Keep your head in contact with them and you should end up past their guard.
Style: Brazilian Jiu-Jitsu
