To better engage the middle use the tops of your toes rather than the bottoms to balance on.
Once you’re ready to descend, keep your butt and belly squeezed while lowering. Keep tight throughout the movement and don’t let the shoulders or middle sag or roll forward. Strongly externally rotate the shoulders to create tension and stability in the shoulders while performing the movement. Drive back to the starting position without internally rotation or dumping. Makes sure to reset and brace each time.