Using custom error pages in Joomla is fairly simple.

First you should create your custom error pages - you can use a text/HTML editor of your choice (for example Dreamweaver). Once you are ready with the pages, upload them into a subdirectory of your public_html called errors. Then open your main .htaccess file and add the following lines in it:

# serve custom error pages

ErrorDocument 400 /errors/400.html

ErrorDocument 401 /errors/401.html

ErrorDocument 403 /errors/403.html

ErrorDocument 404 /errors/404.html

ErrorDocument 500 /errors/500.html

NB: To avoid your custom error pages from being ignored by Internet Explorer, ensure that they are larger than 512 bytes in size.