Well, yours is not one of the simple ones to respond to...but, I can definitely give you a generic answer. If you need more details, don't hesitate to post back!
First of all, to prepare your own wine tourin, I would say to do some studying from home, providing that you have the time to do so...
You would need to get ahold of a good wine map of Tuscany to give you an idea of where the interesting regions are, and where to go...
Basically, some basic knowledge of the are would be a good thing... There are several on-line courses you could take (among which I believe Wine Spectator does one on Tuscany) or you could acquire a book on the subject....there are several.
It would then be good to contact individual wineries in the regions that you want to visit. Each winery is different, but appointments made by you would give the message to the winery that you are really interested in visiting them.
You could also try to go as the wind blows, but you are likely to find closed doors or be disappointed in other ways, if no one's in to speak English, or if they happen to be doing something else that day, etc., etc. So I always recommend an appointment done either by a simple phone call or an email. That's the way I wine-travel myself.... Please remember though that still a lot of businesses in Italy are not very prompt with their emailings (as I was not very prompt to answer your post). However, we usually get to things sooner or later, so be patient!!!