Exit the Stazione Centrale (Central Station) and walk north to Pallazzo Victorrio Emmanuel. Continue north on Via Crispi and continue walking until you reach Ponte Solferino. Cross the bridge and keep walking north on Via Roma, which will take you to the Campi dei Miracoli. This walk should take about half an hour.
Take any one of three buses from the Stazione Centrale: bus line 3, bus line 4 or shuttle bus A. Ask the driver for the nearest stop to Campi dei Miracoli. This should take 10 to 15 minutes.
Take the subway to the San Rossore train-station stop. Exit the station at the Piazza Fancelli side and walk along Via Andrea Pisano until you reach the Campi dei Miracoli.