Log into cPanel on the account to add the subdomain on.
Click "Domains" under the "Domains" section.
Click the "Create A New Domain" button.
ENter the subdomain name to add in the "Domain" text box.
Deselect the "Share document root (/home.username/public_html) with "domain.tld"." option.
Enter the directory where you want the files for this subdomain to exist. (e.g. /public_html/sub.domain.com)
Click the "Submit" button.