Mount, bottom
Upa
The trap-and-roll escape from mount. Take away their arm and leg on the same side so they have nothing to post with, then bridge them over.
Step by step
Trap the arm
Overhook one arm at the elbow, or grab the wrist, and pin it to your chest.
Trap the foot
On the same side, step your foot over the outside of their foot so they can't post the leg.
Bridge over the shoulder
Drive your hips up hard and bridge over your shoulder toward the trapped side.
Land in their guard
Roll with them and come up on top in their guard. Posture up straight away.