Same neutral setup as seen with a split squat lunge. Knees at 90 degrees. Upright and neutral posture. About shoulder width or slightly less foot spacing. Carefully place back leg on stability ball with foot plantar flexed. Consider using a wall, chair, dowel or spotter to initiate and exit the position safely. Lower in a controlled manner. The range will often be limited by tight hip flexors/quads in the back leg. Use quads/gluts/hams to drive back upwards. This motions should be within pain free range.