I've never done this, but isn't it just to create a folder a notch up from your /www directory? If you've got a domain you should be able to create unlimited sub-domains if you want. Don't know what kind of hosting setup you have though.
It depends on his hosting company... Mine works this way: go to HELM to create a sub-domain, and then the folder "magically" appeared under my wwwroot (for example, wwwroot/newsub). From then, I can access that folder by "newsub.domain.com".