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.
Make sure you keep your spine neutral with a natural low-back arch, with shoulders down. Looking forward, not at the ground, will help you avoid rounding your back.
Keep the barbell close to your legs, almost touching them.
Squeeze your glutes to pull yourself up at a quicker pace than it took to bend down. Don't use your back and do not round your spine.
You should be using a weight where you can do three sets of 12 to 15 reps before fatiguing your muscles — but remember that you should be able to still do the move correctly on your last rep.