HOW: Lift your leg and opposite arm up in a "marching" position. Squeeze your glutes and quadriceps and stand tall on one leg with your core braced. You should feel very steady, like if someone came to knock you over they couldn't do so. After holding this position for a few seconds, switch legs and repeat.
FEEL: You should feel all the muscles in your legs, but especially your core and glutes working to keep you stable and from falling.
COMPENSATION: Do not let your hips sink. Your hips should stay level the entire time.