There are two modes of operation for an additional domain, an Addon Domain which acts as a completely separate domain and an Alias Domain which acts as an alternative domain to your existing domain name, with an alias you can use two or more domains on the same website.
To add additional domains you first need to change the nameservers of your domain to:
Setup an Addon Domain
To setup an Addon Domain head over to your cPanel and click on the Addon Domains button. You will see a page with several input boxes.
You can fill the input boxes like in the example image, replace example.com with your own domain name. Please note that the Document Root is the folder where you will place your website files. After you click Add Domain the system will tell you if it was successful. If the domain nameservers are not updated yet (it takes up to 24 hours) the process of adding will fail.
Setup an Alias Domain
To setup an Alias Domain head over to your cPanel and click on the Aliases button. You will see a page with one input box.
Write your domain name into the input box and click Add Domain, this domain can now be used as an alias or alternative name to your current domain.