WordPress.org

Do I need a Google Account?

Yes.

Do I need a 404.php template file?

No, one is included with the plugin.

My 404.php page isn't being served for 404 Not Found errors!?

Add this to your .htaccess file -- and read my .htaccess Tutorial for more information.

ErrorDocument 404 /index.php?error=404 Redirect 404 /index.php?error=404

Fixing Status Headers

For super-advanced users, or those with access and knowledge of Apache .htaccess/httpd.conf files you should check that your error pages are correctly returning a 404 Not Found HTTP Header and not a 200 OK Header which appears to be the default for many WP installs, this plugin attempts to fix this using PHP, but the best way is to use my .htaccess trick above. You can check your headers by requesting a bad url on your site using my online Advanced HTTP Header Tool.