Trunk control in sitting on a gym ball ('Baby development' Balance, sitting, circles on stability ball)

Kneel on the floor holding your baby with the gym ball in front of you.
Sit your baby onto the ball.
Support them around the hips, trunk or chest depending on their requirement.
Roll the ball in a slow circle.
Your baby should be trying to work to stay sitting upright.
Try circling the ball in both directions.