Begin by placing your feet as wide apart as you can comfortably balance. Relax your neck and shoulders. Place your hands on your waist and slowly bend your knees to drop into a 'horse riding' stance. Aim for squatting as close to the ground as you can. Hold the squat for 1-2 seconds then slowly return to standing straight.
Repeat the squat 20-30 times, or as your strength will allow. It is better and more challenging the more slowly you do this exercise.
Eventually you will build up leg muscles and increase your stamina to the point where you can hold the squat position for ten seconds or more.