Custom error pages can be created from cPanel > Error pages.
Once you create them you should create a .htaccess file in the public_html directory containing one of the lines below. If you already have a .htaccess file please add these lines:
For custom 404 error page:
ErrorDocument 404 /~cpanel_username/404.shtml
For custom 500 error page:
ErrorDocument 500 /~cpanel_username/500.shtml