Visitor Segmentation using Google Analytics to create custom segments is covered by an earlier video by Ian Lurie over at Conversation Marketing. Google has an Advanced Visitor Segmentation guide which also covers Visitor Segmentation in detail.
However, I learn best by seeing and hearing and while I never used custom variables for visitor segmentation, I immediately understood what Ian Lauie said - whereas, if I had read it, it would be much harder to grasp, I feel.