This page is inspired by the document found at http://www.ics.forth.gr/dcs/index_main.php?l=e&c=511
Buy 2 tickets, 1.5 euros/ticket. The vending machine only accepts coins for single ticket purchase, and 5 euro bills if more than two tickets are bought. It seems the ticket can be bought from the driver for 2 euros, but I have not tried this option. Handle the ticket to the driver, he will tear one half apart and leave the other half to you.
Bus 11 stop is located at Plateia Eleftherias (Eleftheria's Square) in front of Astoria Hotel (not really obvious, NOT the bus stop to the airport but just a few meters away under the hotel porch): GPS coordinates 25.136799E longitude, 35.338141N latitude.
The trip lasts 25 min around 8:30 AM in mid-August (must be dependent on traffic).
As described at http://www.ics.forth.gr/dcs/index_main.php?l=e&c=511, “the first and very large building of the campus that you will see in front of you and to the right hand side of the road is the University Hospital (“∏AΓNH”); approximately 800 m after that stop, and immediately after a downhill road and a very sharp 135-degree right turn, is the bus stop where you should get off and walk to the left (North)”. For IMBB, stay until the next stop, which is right in front of the Physics and Biology building.
GPS track and pictures along the trip at this google maps link.