How Can I Create A Custom 404 Page?

First, create a new Article called something like "Error 404". In the body, explain that the requested page was not found and provide links to the main sub-sections of your site to help the visitor track down the page they were looking for.

Joel Spolsky had the great idea of adding "(404)" to the keywords of the most popular articles on your site, and then using CityScript to present a list of those articles:

{$foreach x in (keyword_contains "(404)")$}

...

{$next$}

Then you'll just need to know how to configure 404 pages on your web site host. If you have a control panel (such as the popular CPanel), it'll be fairly easy to configure. If you don't have a control panel, check out this site for a possible solution.