Q on setting up site URL structure

I've finally managed to get one of my importers to work, so it's time to start setting up site structures. I have a bunch of blogs (sites in 3.0). They fall into three main categories: sites (sites that are more Web site than blog), blogs (sites that are blogs) and user (blogs that are guest blogs for some of our users).

We have one main site and we'd like to have the URLs fall into the following format (let's say blog 'a' and 'b' are for basic web sites, blog 'c' and 'd' are for main blogs, and blog 'e' and 'f' are guest blogs). Here's how we'd like the URL structure:


a, b, c, d, e, and f are the actual blog name (site names in 3.0).

So, any pointers on how I can set this up?