The Prague underground (subway) is called METRO; it consists of three lines: A: green, B: yellow, C: red. Both the conference venue and your hotel are very close to the metro.
The airport is called LETISTE RUZYNE (Letiště Ruzyně). The relevant metro terminals are called ZLICIN (yellow metro line B) and DEJVICKA (green metro line A).
Two buses commute from the airport to the metro terminals:
You’ll need to buy a ticket on the bus from the airport and another ticket before you enter the metro station in one of the ticket-vending machines (one ride: CZK 26, one day: CZK 100). For that, you will need Czech crowns (CZK). You can either change your currency into CZK at the airport, or you can simply withdraw Czech crowns directly from your bank account in one of the ATMs (cash dispensers) at the airport.
To get from the airport to the conference venue, take bus no. 100 from the airport to the metro terminal station ZLICIN (Zličín) – yellow metro line (B). The yellow metro line B will bring you directly from ZLICIN to NAMESTI REPUBLIKY (Náměstí Republiky, 13th stop), which is very close to the conference venue.
To get to Kolej Petrska, take bus no. 100 from the airport to the metro (= underground / subway) terminal station ZLICIN (Zličín) – yellow metro line (B). Get off at NAMESTI REPUBLIKY (Náměstí Republiky, 13th stop), and walk to Kolej Petrská.
To get to the hotel in Dlouhá Street, Prague 1 (Old Town), it is easier to take bus no. 119 from the airport to the metro terminal station DEJVICKA (Dejvická) – green metro line (A). From there, the third metro stop is STAROMESTSKA (Staroměstská), where you get off and walk to the Old Town Square (Staroměstské náměstí) and to Dlouhá Street.
To get to the AMU hotel, take bus no. 119 from the airport to the metro terminal station DEJVICKA (Dejvická) – green metro line (A). The second metro stop is MALOSTRANSKA (Malostranská), where you get off and walk to the AMU hotel.
To get to the ATELIER 12, you take bus no. 119 from the airport to the metro terminal station DEJVICKA (Dejvická) – green metro line A. The first metro stop is HRADCANSKA (Hradčanská), where you get off and walk to Atelier 12.