From the airport: take bus no.119 to 'Dejvicka', which is the bus terminus and tube station. From there you go straight to 'Mustek' on line A and then change to line B to 'Narodni trida'.
From the main train station: From there you go straight to 'Florenc' on line C and then change to line B to 'Narodni trida'.
A bus/tube ticket will cost you CZK 20 and is valid for 75 minutes and can be purchased from a kiosk at the airport or from automatic machines at the bus stop with change. The journey to the hostel should take approx. 20 minutes from 'Dejvicka'.
GPS: 50°4'55.58"N,14°25'2.25"E