In order to create a new subdomain in your Cpanel hosting plan you will first need to log into the server.
https://yourdomain.com:2082 <- yourdomain.com is replaced with your domain name
Once logged into your control panel scroll down to the Domain options and click on "Subdomains"
Next you will need to enter the subdomain name and it will be automatically assigned a document root, there is no need to change that. Once that is complete click the "Create" button. After that you have successfully created a subdomain in Cpanel.
Note: To create a subdomain, cPanel asks for the name you wish to use. MAKE SURE the subdomain name you give it doesn't have a folder already named that name. If it does, the subdomain, will not work.