How do you adjust your balance cushion?
First, it is important that you don't feel the ground when you sit on your cushion. If you can feel the ground, inflate the cushion using a pump with a needle (sold separately - reference: 8556016).
The air pressure inside the cushion is what determines the instability level. The more your disc is inflated, the less you'll work your balance; the less it is inflated, the more you'll work your balance.