They will do the emergency drill prior to departing port - typically this happens an hour prior to the scheduled departure time.
Really, all you have to do is show up at your designated muster station and pay attention to the instructions they provide. Your muster station location will be noted on the inside of your cabin door and I believe it is on your cruise card too. They will also make public announcements at intervals prior to the drill, telling you what to do. Usually takes 15-20 minutes. We usually go early so we can get a seat. Princess muster stations are in various public rooms around the ship, such as lounges, show rooms, etc.