Holding the barbell (or two dumbbells at your side), keep your arms straight and knees slightly bent.
Slowly bend at your hip joint, not your waist, and lower the weights as far as possible without rounding your back, which should remain straight. Looking forward, not at the ground, will help you avoid rounding your back. Keep the weights close to, almost touching, your legs.
Squeeze your glutes to slowly pull yourself up to complete the rep. Be sure not to use your back or round your spine!