You should squeeze or pinch the ball in between your thumbs and your fingers at a slow and heavy pace alternatively. While in the case of thumb exercises, the main focus should be on your thumb fingers. Try rolling the ball in between the periphery of your thumb. This exercise helps you to get a good control over your thumb and also toughen your thumb joints. https://maxsplace.info/effective-hand-therapy-ball-exercises-to-enhance-dexterity/