Custom Error Pages for a Website

Heya, one of my friends has a website and was wondering how to make custom error pages. I've seen it done before, but have no clue how it works and figured someone here would know. Any help or pointing to information would be much appreciated.

This may or may not help. Depends on your server type & if you can access certain files.
http://www.outfront.net/tutorials_02...tomerrors1.htm