The first thing that we need to do here is the anticipation of your opponent’s movement. As soon as his top leg lands in mounted position, immediately grab it with your hand and lift it up, so your bottom leg can scoot underneath the ankle and your top leg can trap it and secure quarter guard. In this position, you must act right away, because if you stay your opponent will just secure mount again.
So, you must push the knee down and switch your hips in the opposite direction, which will place you in half guard position. From there on, it is up to you if you will be fighting for the underhook and attack from half guard or you prefer doing a simple elbow escape back to full guard.