Here's how to do a bicycle crunch:
Lie face up on a mat in a crunch position.
Crunch up and turn your right shoulder toward your left knee.
Lift your left foot off the floor at the same time and bring it forward toward your head.
Also, extend your right leg straight off the mat.
Hold yourself off the mat in this rotated position for 2 seconds.
Lower your upper body back to the mat but keep your legs lifted.
Crunch up again, but twist your left shoulder to your right knee as you bend your right knee and bring it toward your head.
Extend your left leg straight.
Continue alternating until you have done 10 reps per side.
Keep your lower back flat throughout this exercise.