I must agree with above that it's not that close to Umbria. Having said that here is what we do.
1-buy a good map of just Tuscany and Umbria
2-read up on both and list all the places you would like to see (there will be many more than possible, but that's ok)
3-on the map put a little dot or sticker of the locations you want to see
4-now look at the map and group them by day trip in a way that they are group in such as North, south, east and west of where you are.
5-now finalize what is possible and eliminate what is not.
6-then do one area per day
Further to Bonjours step by step, at step 3 if the locations are scattered find a spot central to those for your stay to reduce travel time. Conversely I will sometimes find accommodation that appeals to me and then draw a circle out from there to determine places to visit that are within travel times I set.