Flight
The Florence airport is about an hour drive from Casa Cornacchi. Keep in mind that there are no direct flights from the New York/New Jersey area outside of the summer season, so you will need to make a connecting flight if you wish to fly into Florence.
Rental Car
Car Rentals are located at “Palagio degli Spini”, connected to the Florence airport by a free shuttle bus that runs every 20 minutes. The shuttle bus stop is inside the short term car park, just outside the terminal. There are multiple car rental companies to choose from, you can visit the website for more information. I would recommend renting a car because the venue is very secluded and it's difficult to find taxis.
Flight
You can fly direct from the New York/New Jersey area to Rome. From there, Casa Cornacchi is about 2.5 - 3 hours away via car. You also have the option to take a high speed train from the airport to Siena taking about 3 hours. You can then get a taxi from the Siena train station to Casa Cornacchi which is about 30 minutes away. If you would prefer to rent a car, there is an Avis located 2 minutes away (driving) from the Siena Train station.
Train
In the airport, there is a non-stop service dedicated exclusively to airport passengers to/from Roma Termini with departures every 15 minutes and travel time of 32 minutes.
Train
From the Roma Termini station, you can take a train to Siena which will take about 3 hours in total.
Rental Car
There are multiple car rental companies in the area near the Siena train station all less than a 10 minute taxi ride away. Enterprise has the best reviews and is located 7 minutes away (driving) from the train station.