Begin standing with your feet shoulder width apart and hands a little wider than hip width apart on the bar
Now slowly lower the bar towards your feet whilst keeping your back and legs straight, the movement should be in the hips and NOT in the knees or by arching the back
Slowly straighten your body again through your hips, keeping your back and legs straight
Repeat steps 2 and 3 until you can no longer maintain correct technique
Tips
Keep your back and legs as straight as possible, you should feel your hamstrings stretch as you go down