From Fiumicino 'Leonardi da Vinci' airport:

Take the first train to Trastevere Station.  From there, take the first train going to any of these stations: Civitavecchia  - Viterbo -  Bracciano - Cesano.  Get off at the first stop (S. Pietro Station) The B&B is 100 metres from the station.

From Ciampino airport:

Take the bus either to Ciampino train station or to 'Anagnina' underground station (this bus runs approximately every 40 minutes Mon-Fri, every hour weekends/holidays).    From there, take the first train to Termini (Rome's main station) and then take Bus 64.     Get off at Via del Crocifisso.   The B&B is 10 metres from the bus-stop.

From Termini train station:

Take Bus 64.   Get off at Via del Crocifisso.   The B&B is 10 metres from the bus-stop.

By car:

From the GRA (Grande Raccordo Anulare)  - Rome's ring road -  take the Via Aurelia (n° 1) exit for the city center, then follow the directions to St. Peter's.