How Do New Users Register And Set Up Their Own Site On The Main Domain But Pull Clones From A Differ

Here’s what I’m thing to pull off:

New users purchase their site at http://www.Domain.com and then create a blog which is cloned from an existing template at http://www.Domain.com/maintheme .

The goal is for each user to have a custom directory like this: http://www.Domain.com/JasonBurtSite and NOT something like http://www.Domain.com/mainsitethem/JasonBurtSite/

So far I have all the multisite functions installed on both the main site as well as the directory (ie http://www.Domain.com and http://www.Domain.com/maintheme)

How do I hook all this stuff up?

What goes where?

Where and How do I set up the cloning function so that when new users register sites they are autocreated from the correct install AND end up just off the main domain?