Sit down on the BOSU with your trunk upright and not bent. Bring your legs at shoulder width and lean your heels on the floor. Your thighs and calves make an about 90˚ angle at your knee joints. Your arms are alongside your body, bent at the elbows at a 90˚ angle and your palms are laid loosely on your thighs.
Slightly bend your trunk back but do not bend your spine. Keep the fixed position, without changing the knee, trunk and elbow angles. Maintain 90˚ angles.
Pay attention to your body posture, fixing your body core and keeping your pelvis in a neutral position and avoiding over-bending your spine.
Keep your knees in the direction of your toes, at shoulder width all the time.
Keep your arms in a neutral position and do not move them forward.
Exercise in front of a mirror to watch and improve your technique.
To make the exercise more demanding, you can push your hands against both thighs.