1) First you have to update the nameservers of the domain which you want to addon to your main domain nameservers

2) After updating the nameservers it takes 24 - 48 hours for dns propagtion.

To add this through your cPanel. Choose the menu option "Add-On Domains"

New Domain Name: the name of the domain (ex: yourdomain.com)

Username/directory/subdomain: the FOLDER you wish for the domain to point towards (ex: yourdomain). The word in this text box will be used for username, directory and subdomain.

NOTE: simply the name of the folder you want the domain's content to be stored in, will be sufficient. It can be a folder that already exists or make up a name and it will create the folder for you.

Password: password for your account

Ftp user will be username@domainname.com with the password you are creating here. This will allow a new ftp user access to this directory only.

Be sure to wait a couple of minutes for the page to load after you create them. It takes a few minutes from when you click on add for a confirmation screen to appear.

Be sure to update your name servers for your addon domain to point to the same nameservers as your main domain.