You should use a CNAME record whenever you want associate a new subdomain to an already existing A record; i.e. you can make “www.somedomain.tld” to “somedomain.tld”, which should already have been assigned an IP with an A record.
You can add this from the cpanel of the domain itself.
1. Log into the cpanel of the domain.
2. Click on the option “Simple DNS Zone Editor” listed under Domains.
3. From the drop down menu select the domain for which you need to add the CNAME.
4. When you scroll down you will have an option like “Add a CNAME record”
5. You can specify the CNAME