For demonstration and additional explanation, click the link:
Stand with feet approx. shoulder width apart.
Stand up straight with ears lined up above shoulder, lined up above hips, lined up above knees and ankles.
Brace the core by inhaling through the nose, breathing diaphragmatically (with your belly, not your chest), and upon exhalation create approx. 20% tension in the area between your ribs and pelvis (360 degrees, your core). You are now braced. This does not mean to hold your breath, but a little bit of tightness and tone in the abdominal region. For some bracing and being able to breathe while bracing may be the starting point. This should be the staring point for all subsequent movement; the air squat and beyond.
Begin the movement by hinging at the hips, with the hip being the fulcrum and lever arms being your femur (thighs) and torso (abdomen and chest). Both of the lever arms should remain as straight as possible throughout the movement. The hip hinge is executed by sticking out your backside and pushing it posterior. You can counter balance by increasing your hinge and keeping your arms out in front of you (as seen in the demo above).
Ideally we want to work on maintaining the braced core, maintaining a straight torso (if not slight extension) and lowering your backside down to the point where the hamstrings are parallel to the floor and the shins are perpendicular to the floor. Once you reach your personal depth, you should be able to pause, and then in a controlled fashion, rise back up while maintaining balance, and the straight spine.
That would be ideal. However, we all have our personal baselines and starting points. Find yours and work at improving it. It may just be getting comfortable with bracing the core and beginning the hinge. That is fine, that is the start of proper movement which, with practice will protect you and provide power as we move through life each day.
If you notice that the knees translate forward as you descend, to the point where they are becoming even with or moving anterior to the toes below, you have an anterior dominant squat which can wreak havoc on the knees and neglects relying on the optimal intended core and glute strength. Do not proceed to drop deeper into the squat until you can do so without knees staying behind the toes. Once you have improved balance, core stability, and activation of the glutes, you will be able to continue your decent correctly.
The “Chair of Death” exercise shown above can be implemented as a means to keep you in check as you practice mastering this essential, functional movement.