What is knee on belly? how do I get there? Here are some answers.
(from side control, your left arm under their head.)

1 – Get thumb in grip with your left hand behind opponents neck.

2 – Put your right arm up across their hips, apply pressure down. Quickly slide your right leg up across their hip and replace your arm with your right shin to achieve the knee being on the belly.

3 – Grab far knee with your far hand. Right hand grabs the outside of their left knee.