Lie on your back with your fingers behind your ears and your legs raised at ninety-degrees at your hips and knees. Think of it like sitting in a chair while lying on your back.
The idea with the bicycle is to try touching your left elbow to your right knee and your right elbow to your left knee. To do this you will crunch upward while twisting at the waist to send one of your elbows forward while drawing the opposite knee toward your elbow.
TIP: Try not to help close the gap between your elbow and knee by just swinging your elbow over. Instead, really use your abs (obliques) to twist your torso to help close the gap.