Sit on the swiss ball (try and find a ball the right size so that keeps your thighs almost parallel to the ground when in sitting.) Rest your feet hip distance apart with your toes facing forwards. Find neutral spine by tilting your pelvis forward and backwards. Make the movements smaller until you are rested in neutral spine. You should be sitting directly on top of your sitting bones. Draw shoulder blade towards buttocks and gently push one heel into floor. Slowly take the weight out of your leg and lift your foot off the floor. (DON'T your pelvis rock sideways or tilt backwards). Hold shoulder blade, tummy and buttock ON. Practice breathing for 5 secs, then lower leg back down. Repeat with the other side, do more on weaker side.
PROGRESSION: Try balancing on the ball maintaining neutral spine with your feet off the ground.