1. Login to cpanel and select Addon Domains under Domains category.
2. In the first field 'New Domain name' give your domain name without 'http' or 'www'. Just give the domain name and its extension.
3. In the next field Username/directory/subdomain Name, you must give a new name to the folder or to the new directory that you are creating inside the public_html directory for the new Addon domain.
4. Now you must create new password. As usual you can go with password generator and the password which you are creating is the FTP password for the account.
5. Then finally click Addon Domain button which paves you the way to your new Addon Domain.
Now you may know How to create an Addon Domain with your Cpanel. So after this you would be able to upload files into the new folder that you have created for the new Addon Domain.