HOW: Start by lying on your back with your knees bent. Engage your core by drawing your ribcage down. Bring your shoulders and upper back away from the floor with the goal of bringing your chest to your knees. You may need to wedge your feet under some sort of sturdy bar/device to keep your feet down. Then lower yourself and repeat.
FEEL: You should feel your core and abs working to lift your upper body off the ground.
COMPENSATION: Keep the movement as slow and controlled (it's hard!)