Bus service

We assume you did your research so Just do it. Nothing to it.

How often do the busses run.

Go to the Tourist information center or ask the check in people at the hotel.

If you go out the side door of the train station, you'll see McDonald's Restaurant. The buses are lined up to the right and on the street behind McDonalds.
They'll take you into the city center while the train drops you on the outskirts of town. They're also quicker.

It's easy, takes about an hour. Buses are frequent. The bus stop in Siena is Piazza Gramschi.