This guide will go over:
* Making sure your theme is “widget friendly”
* Creating a functions.php file to register the sidebar
* Enclose your static sidebar in the dynamic sidebar conditional tag
* Making multiple widget-ready areas
* Other creative ways to use widgets
Link: http://www.themelab.com/2008/04/18/see-how-easy-it-is-to-widgetize-wordpress-themes/