First, lie flat on the floor with your lower back pressed to the ground. Put your hands behind your head. Bring your knees up to a 45 degree angle and let your feet come up off the floor. Slowly go through a bicycle motion with your legs. Extend one leg as you bend the other leg and bring it towards your body. Then alternate sides. As you “pedal” your legs, you will reach your left elbow towards your right knee (as it comes towards your body), and then your right elbow towards your left knee (as you switch sides).
As you do the exercise, keep your abs pressed towards the floor! Go slow enough that you can breath evenly throughout the exercise and maintain correct form. (As with most ab exercises, form is more important than speed!)
Keep your eyes focused at an upward angle. This will prevent you from pulling your neck and head too far forward. By keeping your neck relaxed and straight, you can focus on using your abs to do the crunch. Better form means better results.
With the bicycle crunch, the amount you extend your lower leg will determine the difficulty of this exercise. If you are advanced, you can extend the lower (extended) leg until it is completely straight. If you are a beginner, you can keep the extended leg slightly bent. Regardless of your level, keep in mind that it is more important to maintain correct form and do an easier version of the exercise. Once you are stronger and can maintain proper form, you can increase the difficulty of the crunch.