If you host your own DNS server you must do it yourself. If you don't then your DNS provider should do it. You will do it where you configured the IP address record pointing your domain. An "A (Host) Record" must be created.
For example, you must have a record like this for your main domain name to server as a web site:
85.110.100.200 A domain.com
If you can reach your domain using
www.domain.com then you already have a record for
85.110.100.200 A www
Now you need this:
85.110.100.200 A subdomainname