How to set the custom 404 page in joomla?
or
Change default “Joomla 404 component not found” page

Ans :

Create 404.shtml file under the document root then open the error.php file

vi   /home/username/public_html/templates/system/error.php

then add the following code below the line  “defined( ‘_JEXEC’ ) or die( ‘Restricted access’ );”

=======================================

if (($this->error->code) == '404') {
echo file_get_contents('http://www.domain.com/404.shtml');
}
else
{

==========================

And then add the following code at the end of error.php file

=============

<?php
}
?>