You can perform the exercise with the bar on your shoulders or on your chest. Lets show how its done traditionally on the back. With your arms grabbing the bar at a wide distance, lower the bar onto your shoulder blades. Your lower back should be slightly arched and you should tense your core.
Start the movement by pushing your hips slightly back and then bend your knees. Your back should stay arched all of the time you are moving both down and up. When your thighs are parallel with the floor, then slowly move back upwards
Keep your upper body as upright as possible and never slouch the shoulders forward. Keep your eyes fixed on something at head height and don’t look down at the floor. As you drive yourself back up, push your heels into the floor.