So, so much…
I’d start with some warm-up walk, trot, canter both directions with your hands on your hips. Focus on the lower back being loose, and the hips following. Stand in the stirrups to stretch the leg and heel down.
Then I’d add some movements to practice independent hands. 3 strides on your hips, 3 stride straight out to the side, 3 strides straight out in front, 3 strides straight up over your head, repeat. Then try the YMCA or the Macarena.
Then I’d take away the stirrups (in addition to reins), and do a few circles with your hands on your hips to focus on loose, relaxed, and following.
Then I’d add some movements to practice independent seat. Lift both legs away from the horse for 2 strides, then let them relax and drape down, repeat. Scissors - right leg back, left leg forward, then switch. Knee bends - with your leg long and draped, bend the knee as if you want to bring your heel up to your butt. Alter the angle of your upper body without changing your leg - lean back for 3 strides, vertical for 3 strides, a bit forward for 3 strides. Arms straight to the sides, rotate 90 degrees each direction in your waist, keeping hips forward. Toe touches on the same side, and on the opposite side. Touch the poll and the tail.
When the rider is really solid, you can practice affecting the horse - bumping with the lower leg to add jump while still following with the seat, closing the upper thigh to half halt, crunching the abs to weight the hind leg, etc.