I will advise you to travel with a Sat nav. A brief description on how to get to Bagnaia from Roma Fiumicino is has follow;

From the airport follow for Rome, then take the exit for Citta Del Vaticano.
From there bare left and follow signs for Viterbo, this will take you to the Circular road or "Grande Raccordo Anulare".
Stay on the Circular road until you reach the exit for Cassia Veientana Viterbo, follow the Cassia until you get to the exit for Viterbo. From there Bagnaia is only 4km away.