To create a custom error page for your account you will need to upload a file with the appropriate file name to the account.
The file will be named according to the HTTP error code the page corresponds to. To create a custom page for 404 errors you would upload a page named 404.shtml. For a 403 error it will be named 403.shtml The server will automatically recognize the custom error pages for the primary domain name on your account if the file is named with a .shtml extension.
Setting a custom error page from the cpanel or by manually editing the .htaccess file for the root domain will apply that error page for all domains on the account. To create separate custom error pages for an addon or sub-domain you will need to create a .htaccess file in the addon or sub-domains folder with an 'ErrorDocument' command.
The format for the command is: ErrorDocument *Error Code* http://domain/page
Be sure you replace *Error Code* with the specific error code you are referring to.
For example if you are creating a 404 page you will need to replace *Error Code* with just 404 For an addon or subdomain the error page path must be a fully qualified URL. You can use the ErrorDocument command to redirect page errors to a file name with an extension other than .shtm.