Zercher squat

Set a barbell in a squat rack at about the height of your stomach. Place the bar securely in the crook of your elbows, clasp your hands together, and lift it from the rack.
Take a step back into your normal squat stance. Keep the bar held tightly to your torso and your core braced.
Initiate the squat by sending your hips back and down. Focus on keeping your chest up and your torso as vertical as possible as you descend.
Go down until your hips are below your knees, or as deep as your mobility allows. Drive through your feet to stand back up powerfully to the starting position.

Tips

Keep the bar pulled firmly against your torso throughout the movement so it stays stable in the crook of your elbows rather than drifting forward.
As you descend, focus on keeping your chest up and your torso as upright as possible, which is the main postural demand that distinguishes this squat from other barbell variations.
Set the bar in the rack at a height that lets you load it into your elbows without having to bend down awkwardly or rise onto your toes to unrack it.

Letting the elbows drop as you descend causes the bar to roll forward and down out of the crook of the arms, breaking the secure hold on the bar.
Allowing the torso to fold forward excessively during the descent, rather than staying upright, changes the bar path and places a greater demand on the lower back than the exercise is intended to produce.