This is an excellent exercise for not only strengthening the lower body but also to help develop the postural muscles around the shoulders and upper back.
Start with the feet hip width apart with toes pointing forwards.
Position the arms directly above the head with the biceps against the ear (you may find it easier to hold an exercise ball above your head)
Squeeze the shoulder blades together and down, and hold in this position
Squat down as far as comfortable ensuring the pelvis stays neutral throughout and the heels stay on the ground
At all times ensure the biceps stay close to the ears and your arms stay above your head
Return to the start position slowly and repeat for the desired number of repetitions
Concentrate on keeping the spine lengthened throughout the exercise