SQUAT: BRACING
To brace for the squat first contract the muscles throughout the core, including the front, side and back. Engage the glutes and think of shortening and thickening through the waist. Do not round the back . Similarly you can use a belt and put that same pressure into the belt. You should see 360 degrees of pressure pressing out into the belt as you brace. These contracted muscles as well as the pressure will support the spine.
For your squat, keep your brace as you bend the hips, knees, and ankles. Maintain this pressure throughout the whole squat, including the bottom. When you squat look for breathing down into the core before your squat. Signs of breathing incorrectly include the chest or rib cage rising up when you squat or if you see the bar moving upwards.