Start by lying flat on your back on a mat with your arms by your sides and your palms down. Extend your legs fully out with a slight bend in your knees. Lift your heels about 6 inches off the floor. Make small, rapid up and down scissor-like motions with your legs. The key is to focus on having your midsection do all the work and to keep your abs constantly contracted throughout the exercise.
See video here.