I found in Peter Kirwan's pages the most succinct and helpful advice on what e-portfolio is and how to build it.
Peter also has his own e-portfolio page, which looks brilliant.
There is also another advice page by Dilip S. Mutum
University of Warick also has its information page on e-portfolio making
Perhaps, the best way to learn what e-portfolio is and how to design it is to see what other e-portfoios look like. Here are some good examples I found.