cPanel's way of doing it doesn't function properly on our current hosting setup; open your .htaccess file either with cPanel's File Manager or FTP (usually easier to use cPanel for this), and add the following line(s) to the file:
Code:
ErrorDocument ### Path/To/Errordocument.ext
Where ### is the error code (404, 403, 500, etc), and Path/To/Errordocument.ext is the path to the error document you want to display. If you've made some via cPanel's error page manager, it'll likely be similar to this:
Code:
ErrorDocument 400 400.shtml