Prepare the BOSU with the flat side up. Stand on the BOSU with feet slightly more apart than shoulder width. Fix your posture and pay attention to a neutral position of your pelvis.
From the starting position go down, flexing your knee and hip joints without changing the spine curvature. Guide your knees in the direction of toes during the movement. Keep your body core firm and do not bend forward too much. In the lower position your pelvis gets to knee level. Return up to the starting position the same way. Inhale in the starting position, fix your body core with air in, and exhale returning back to the starting position.
Pay attention to body posture, a firm body core, a neutral position of pelvis and eliminate bending at the waist.
Keep your knees in the direction of your toes throughout the entire movement to avoid knee swinging. Also pay attention to the forward movement of your knees and remember that the knees, trunk and head should not go beyond the fictitious toe line.
Avoid excessive bending forward when going down.
To start, you can exercise holding on to something, if needed, until your body gets used to the unstable platform.