Engage your core, hinge at the hips as you lower the bar, keeping your spine neutral . then drive your hips forward and use your hamstrings to power back to standing position.

Beginner: Try just the bar first or dumbbells 

Intermediate: 10-25lb plates on each side of the barbell

Advanced: 30-55lb plates on each side of the barbell

*Modification: use dumbbells instead