All necessary actions can be performed through your account's cPanel.
It's assumed you already have the new domain name registered and its Name Servers pointing to your hosting account (see orther FAQs on how to do this).
- Login to your account's cPanel. Be default this should be located at cpanel.yourdomain.com.
- Click on the "Addon Domains" icon in the Domains grouping of icons.
- Enter the following pieces of information:
- New Domian Name.
- FTP Username*.
- Document Root (should default to the same as your new domain name).
- Password
- Password Confirmation.
- Click "Add Domain!"
*Your primary/master account will be able to access your Sub-Domain's folder and manage it's content, but by having a separate FTP account/user for this Sub-Domain you can have someone manage the site and only have access to the Sub-Domain's content. This prevents this user from accessing your primary site's folders.

