• By plane:
• Inside the airport, among the car-rental agencies is an MHD (or DP) booth that sells public transport tickets, as well as unlimited tourist passes for 1, 5 or 10 day unlimited travel on ALL Prague public transport. They're a great bargain, as well as a convenience. 14 crown tickets are good for a single ride. 20 crown tickets are good for an hour, including any transfers and will be your best individual-ticket bet.
• The bus stops are just outside the Airport International Flights exit. Take the #119 bus to Dejvicka (the 119's last stop, where everyone will exit). The metro entry is right there. Remember, your 12 crown ticket is still good and you needn't stamp it again, just keep it in your pocket in case you're asked for it. This is also the end of the Green Line A metro, so there's no choice to make but to get on. Take it to the 'Museum' stop, where you change to Red Line C Metro. All three Prague metros (Red, Green and Yellow lines) converge at Museum, so take a moment to orient yourself and follow the signage.
• Your ticket is still valid. Take the Red Line C metro 4 stops to Nadrazi Holesovice and exit using the staircase. You're almost at A&O City Hotel and Hostel Prague---follow the tram tracks to the right about 150 meters, then turn left, following the tracks another 120 meters, under the railroad bridge. Come up the stairs to the green building on the corner - that's A&O City Hostel.