newby question – do I need to setup a whole wordpress blog for multisite use

My apologies in advance for this really newby question.

I have setup WP Multisite – worked fine. I am now trying to setup a sub-domain site for a spanish language version of the site.
Question: How do I do that? Do I need to make a directory on my server and load a new WordPress install into that directory or does WP Multisite do that when I press add new site (so far it doesn’t seem to).

I am sorry. None of the documentation mentions setting up a new directory or wordpress install, it seems to just assume that it just happens..

Thanks for any help, Peter