Start by placing your bosu ball upside down on the floor.  Grasp the flat base of the bosu ball and position yourself as if you were going to do a push-up.  Tighten up your form by drawing your abs in towards your spine and flatening out your back.  Once you have a steady grasp and a balanced form, slowly raise and lower your legs, beginning with the left and following suit with the right.  As you raise your legs, squeeze your glute muscles as tight as you can.

See the short instructional video below.

Your triceps and chest muscles should be engaged as you firmly hold your stance on the base of the ball.  Your core muscles (abs, obliques, etc) are being worked to support your lower back, and you are drawing your abs in towards your spine, contracting and holding the muscle.  Finally, your butt is getting a good blast with the reverse leg raises. 

Try starting with 10-12 raises per leg.  Don’t rest inbetween changing legs.  After you complete 10-12 on each leg, give yourself a 30 second rest.  Perform 3 sets.  Once you become more advanced at this exercise, you can add to the intesity by wearing ankle weights.