From the Barberini Metro Stop: When you come out of the turnstile, go left to exit the metro station. You will see a movie theater on your left, and the plaza on your right. Walk down to the bottom of the plaza to via delle Quattro Fontane. There is a pharmacy on your left at this corner.
Via degli Avignonesi is directly in front of you (it's a small cobblestone street going down from via delle Quattro Fontane.) Go about halfway down this street to number 20. Please note there is no street-front reception and no sign on the building. Look for Daphne on the door buzzer.