Starting Position: Stand with both feet together or spread a few inches apart (2-3 inches) and parallel with each other. Pull your shoulder blades down your back and engage your abdominal muscles to stabilize the spine. Bend your knees slightly and keep tension in your inner and outer thigh muscles. This will help control the tendency to shift 
sideways during the single-leg stand.

Movement: Slowly lift one leg 3-6 inches off the floor; find your balance on the standing leg. Avoid any sideways tilting or swaying in your upper body and try not to move the 
standing foot. Hold for 10-15 seconds before returning the foot to the floor. Perform an equal number of repetitions with each foot.

Exercise Variation: Increase the intensity of this exercise by going through the following progressions: (1) lift the one leg higher off the floor (illustrated) to further raise your 
center of mass, (2) raise both arms overhead, (3) lift one arm to your side, (4) tilt your head, (5) close your eyes, then finally (6) close your eyes and tilt your head.

Source: American Council on Exercise  https://www.acefitness.org/exercise-library-details/1/114/