Assume the same stance and barbell positioning as you do for the barbell back squat.

Descend just as you would for a back squat, but press the bar overhead. It should feel like the bar is pushing you down into the floor. Lower yourself only to the point before your backside starts to tuck in. Maintain the barbell overhead as you press through your feet and return to the starting position. Lower the bar back down while in the standing position and repeat.