Summarize:

For this method, you will need a Swiss ball to sit on, typically between 14 to 20 inches (36 to 51cm) in diameter. Place the ball on an exercise mat, and sit down carefully on it. It will be very difficult to balance on the ball without support, so place your hands behind you on the floor, a bit wider than shoulder width apart. If it is difficult for you to do this, you can also rest your hands on a short bench, placed behind you. You might also try using yoga blocks to rest your hands on. With your abdominal muscles contracted, and your spine straight, lift your legs straight into the air until your body makes a "V" shape. Take a breath in, and as you inhale, slowly open your legs. You should feel the outer muscles of your thighs contracting. Exhale as you bring your legs back together. You should repeat the exercise for three sets of 15 repetitions. Repeat this exercise three to five times each week for best results. If this is very difficult for you, try starting off by sitting on the floor and work up to sitting on the ball.
Sit on a ball. Use your hands for support. Begin the exercise. Repeat.