HOW: Get set-up laying on your stomach with your knee bent at 90 degrees so that your shin is facing up towards the ceiling. While keeping your hips and pelvis flat and parallel to the ground, slowly let your foot fall out to one side followed by falling out towards the other side. Repeat.
FEEL: You should feel your core working a little bit and your deep hip muscles working to slowly control your leg falling in and out.
COMPENSATION: Do not let your entire body move and your hips/pelvis lift off the ground. Work within the range of motion that you can control while trying to keep your low back flat as well