I don't see how figuring it out on your own includes asking someone else.
Anyway, I made a new folder for the new CP, and put the admincp global.php and index.php in it. Begin tweaking.
I think I actually hardcoded the XML parser for the navigation to just read one file, or something like that.
The biggest problem was with the logins, you'll have to create a new login method (for lack of a better term), and specify that in your login form. You'll also have to add a new case in one of the login hooks (check login.php). Make sure you do your own security check and set $checkpwd = 1; if they don't meet it.
I'm still not sure if I did the login session stuff correctly (or did at all, I don't remember), but it was definitely something I am still iffy about. Anyway, you'll figure it out, just might take a lot of tweaking.