1. Log into your Plesk control panel.
2. Click the button labeled "new domain" (Plesk 5) or "Add New Domain" (Plesk 7.5).
3. You will be presented with a screen that will ask you to enter a domain name. Make sure the "www." box is not checked when creating subdomains, then enter sub.domain.ext (replacing "sub," "domain," and ".ext" with appropriate values) for the domain name.

The remaining hosting configuration will be just like setting up a regular domain.