Bicycles work almost every muscle in your core with one movement, therefore they are very effective.
Begin by laying flat on the floor with your lower back pressed up against the floor. Place either hand on the sides of your head but do not pull on your neck. Next, raise your feet off the floor and create a 90 degree angle with your knee joint. Your lower leg should be parallel to the floor. After that, bring in the knee of your left leg while straightening out your right leg; this should be similar to a cycling movement on a bike. As you are moving your legs you should bring your right elbow to your left knee by crunching up using your abs. Now, return to the starting position and repeat by crunching your left elbow to your right knee and straightening out your left leg. Repeat this movement to each side until you have completed your set.
Slow down, many people will go through this motion quickly, but if you go slower you will work your abdominal muscles much harder. Make sure to squeeze your ab muscles as your bring your elbow to the opposite knee.