Iznik is easily reached from Bursa or via the ferries from Istanbul to Bursa. Dolmus marked "Iznik" regularly depart from Yalova, north of Bursa, from the main road that passes in front of the ferryboat landing (cross the street and hop on one headed south) as well as from Bursa itself. Once in Iznik, buses deposit passengers on the main road in front of the church. Drivers should take the car ferry from Istanbul to Yalova. From Yalova, follow the road to Bursa, taking the first turnoff at Orhangazi to Iznik, and follow the road along the north side of the lake.
Iznik retains the grid plan established in its Hellenistic era. Monuments are well signposted, but without a car, you'll be pounding the pavement for the better part of a day.