Creating a Subdomain
Log into Plesk. On the main page you will see a button that says "New Domain" - click it.
Uncheck the "WWW" box.
Type in your new subdomain - as subdomain.domain.com
Click "Update."
The new subdomain will be added to your main page list of sites. The subdomain is managed just like the other sites. You will need to go through and adjust "Preferences" and "Hosting" just like you would for a new domain.
If the subdomain is for a new client, you need to set up a "Domain User" as well.
You also will need to go into DNS and switch the nameservers for the domain.
Remember that the users login name for the control panel will be subdomain.domain.com.
It is a lot easier when you remember that last point!
Wen