Start by finding a good grip on the bar with your hands just outside of your shoulders. You should be able to keep your wrists straight as you step under the bar. The bar should be situated below your shoulders about mid scapula. The placement further depends on where you can raise your elbows while keeping your chest up and wrists straight. This will not be the same position for everyone. Assume a squatting stance with your feet and take a deep breath. Stay in a neutral spinal position and create tension in the shoulders by raising the elbows and tightening the upper back. Right before lifting out of the rack squeeze your butt, bells and drive yourself upright.
Elbows, chest and head shouldn’t drop at any point!
After removing the bar from the rack step back from the rack and reset your feet to a good squatting stance. Begin the descent by re-bracing your butt and abs then lift your shoulders to stiffen the upper back. Keep your shins vertical and push your hips back. Knees should flare out as you descend. The squat ends when you cannot keep your torso up and spine neutral.
Low Bar Back Squat will result in a hip position further back than High Bar and a more forward angle in your torso.
Keep your knees out, feet flat on the floor and remember to brace for every movement.