In a seated position with good posture and feet flat on the floor, cross your arms over your chest or hold a ball with both hands at chest level. Keeping your weight on your heels, stand up, using your hands as little as possible or not at all. As you bend slightly forward to stand up, keep your back and shoulders straight. Take at least 3 seconds to sit back down. Repeat 8 to 15 times or as many as you can comfortably do with good form. Rest. Do another set of 8 repetitions.
Modification: If you are more advanced, try doing squats. Beginning in a standing position with back facing the seat of a chair, slowly bend the knees to lower down toward the seat of the chair. Stick out the buttocks so that your knees do not jut beyond your toes. Just before your buttocks gets to the seat of the chair, stand back up to the starting position in a fluid motion, squeezing your buttocks and putting all the weight in your heels as you push back up.