Once you are logged into your cPanel account:

Select "Addon Domain" under the "Domains" section:

Next, you will need to fill out the details of the add-on domain:
  1. "New Domain Name:" You will put in the domain.com without put www in the name!
  2. "Subdomain:" This will be use for addon/subdomain FTP account username.
  3. "Document Root:" This will be the folder cPanel creates in your main FTP accounts.
  4. "Password:" fill in any password you want here.

Once you are finished filling in the details, select the "Add Domain" button.