For this exercise you will need a long bar normally the ones you use when you do chest exercises, this bar should be on a rack. Put the bar behind your head and on the back of your shoulders and take a step or 2 back. With the weight on the back of your shoulders squat down, lean slightly forward and keep your back straight and come back up to standing position. Do this nice and slowly and make sure you do NOT add weight too heavy as it can result in serious injury.
1st set just the bar 10 reps (beginner)
2nd set 5kg 8 reps
3rd set 5kg 8 reps
4th set just the bar 12-15 reps
Move up by a 5kg every week depending on how comfortable you feel.
[image from trueathelite.com]