Joomla

To create a Joolmla 4 custom 404 page follows the steps below...

Create an article called "404", style it the way you want the page to look including links.
create a hidden menu with a menu item called "404" set the alias to "404"
go to the 'link style" tab and turn off "Display in menu"
go to the alias you just created like http://hallhome.us/404
go to your template, then files then 'error.php' Put this code as shown in the image below.
go to a silly url like http://hallhome.us/burp and verify you see the fancy 404 page you just created. If not, walk back thru these steps again.

/* Load Custom 404 article with true 404 error code returned */
if ($this->error->getCode() == '404') {
header("HTTP/1.0 404 Not Found");
echo file_get_contents(JURI::root().'/404');
exit;
}

To create a Joomla 3 custom 404 page do the following...

Create an article called "404", style it the way you want the page to look including links.
create a hidden menu with a menu item called "404", set your alias to the link you want to use. I recommend "404"
go to "link type" tab and set "Display in menu" to off
go to "metadata" tab and set it to "noindex,nofollow"
go to cpanel and copy the error.php to your template root directory
go to your template files and open the error.php we just copied a few steps ago.
enter the rewrite code, as shown below, replacing "your-alias-here" with the alias we used when we created the menu item.
go to your home page then put a silly url in like http://hallhome.us/burp
If you get your 404 page congratulations your done. If not, go back thru these steps.

/* Load Custom 404 article with true 404 error code returned */
if (($this->error->getCode()) == '404') {
header('Location: /your-alias-here');
exit;
}

If you have SEF off then you'll need to follow the steps below...

Create an article called "404", style it the way you want the page to look including links.
create a hidden menu with a menu item called "404"
on your menu, click the 404 menu item you just created.

copy the link path shown in the url, such as...

index.php?option=com_content&view=article&id=44&Itemid=462

go to cpanel and copy the error.php to your template root directory
go to your template files and open the error.php we just copied.
enter the rewrite code replacing "your-alias-here" with the link we just copied
go to the 404 menu item and on "link type" turn off "display in menu"
go to "metadata" and in the robots area choose "noindex,nofollow"
go to your home page then put a silly url in like http://hallhome.us/burp
If you get your 404 page congratulations your done. If not, go back thru these steps.

How to create a Joomla 4 custom 404 page

How to create a Joomla 3 custom 404 page

**Original Documentation on Joomla.org