Adding a CNAME to cPanel is very easy and fast, here are four simple steps you can follow:
Go to the search bar and write "zone", it should appear the Zone Editor icon, click on it:
Find in the domain list the one you want to modify and click + CNAME Record:
Add the subdomain and the external/internal domain you wish to point that subdomain and click on Add a CNAME Record:
Voilà! You have added a CNAME record.