HOW: Start by lying on the ground on your back. Place your hand's palm down underneath your hips. Brace your core by drawing your ribcage down, which should flatten your back against the ground. This is your base position and you should not lose this position during the exercises. Bend your knees and then point your feet straight up in the air. Maintaining your base position, slowly lower your legs as far as you are able to without losing your base position. You may be able to lower them all the way, or you may not be able to. Then repeat.
FEEL: You should feel all the muscles in your core working, especially your lower abdominal muscles and maybe even your hip flexors.
COMPENSATION: Do not let your back arch! Once you set your base position, you cannot lose that lower back position during the exercise.