Domain as part of existing domain

To Configure the DNS as part of an existing Domain what steps we have to differ from the one in which we just create a new domain?

depends on the dns server software you choose.

Irrespective of what DNS server you are using, you have to know how to create a new record for that zone. Say you're creating a subdomain (server2.mydomain.com) for an already existing domain (mydomain.com), all you have to do is create a new A record in the mydomain.com zone for subdomain.mydomain.com. The actual steps to do this depend on the DNS server software you are using, but if you know how to configure it for a new domain, you shouldn't have any trouble figuring out where to put the new record.