In the video below you can see a step by step tutorial on how to add multiple domains to your cPanel shared hosting.
1. Log in your web hosting account
2. Go to cPanel
3. Under Domains section choose Addon Domains
4. Enter the new domain information
In this step, cPanel will create a folder for the new domain which will be the root folder for the domain. You should put all the related content like CMS files and so on that will be accessible through this domain here.
5. Click on the Add Domain button and your domain will be added to your web hosting account.
This feature is only available if your hosting plan allows multiple domains and if your host runs cPanel.
Also, don't forget to point your domain DNS records to your web hosting address. A video on how to do that you can find here.