Klementinum - also known as Clementinum is situated on the right bank of the river Vltava, the Chapel of Mirrors is located inside. From Charles Bridge you can follow Karlova Street. Klementinum complex is on your left. After about 300 meters walk you’ll see the entrance gate. The Chapel of Mirrors is then inside in front of you after about 100 meters.
Alternatively you can enter Klementinum from Mariánské náměstí Square (eastern gate), after you are arriving from the Old Town Square or metro Staroměstská, respectively tram 17 or 18.