From North:

When you reach Florence take the Firenze Pisa Livorno ringroad in the direction of Pisa Livorno. Exit at Empoli Ovest. Keep on towards Castelfiorentino and Certaldo. At the first traffic light in Certaldo turn left (via Agnoletti) and keep on driving for about 3km following the signs to San Martino

From South:

When you reach Siena take the ringroad Siena Firenze, in the direction of Florence. Exit at Poggibonsi Nord. Drive in the direction of Certaldo. Take the freeway SS 429 until the sixth traffic light. Take Via Agnoletti on your right and keep on driving for about 3km following the signs to San Martino

From Pisa Livorno:

Take the Firenze Pisa Livorno ringroad in the direction of Florence, exit at Empoli Ovest. Keep on towards Castelfiorentino and Certaldo. At the first traffic light in Certaldo turn left (via Agnoletti) and keep on driving for about 3km following the signs to San Martino