There are several tutorials available online to help you with this exercise. The simplest tutorial, which you may want to start with, is here:
http://animation.about.com/od/flashanimationtutorials/ss/flash29walkcycl.htm
Once you are comfortable with the technique, here is another you may want to reference:
http://www.angryanimator.com/word/2008/11/01/animation-tutorial-2-walk-cycle/
This is a great page with examples and tips on creating walk cycles:
http://minyos.its.rmit.edu.au/aim/a_notes/04_walkcycle_project.html#
Here is one specifically using the bone tool, with details on how to make the bone tool work with characters:
http://www.adobe.com/devnet/flash/articles/character_animation_ik.edu.html
And, if you’re into My Little Pony:
http://www.youtube.com/watch?v=Ht2-Xt5-ufw&feature=relmfu