From Bonn central station, take the DB Regionalbahn (regional train, a.k.a. Voreifelbahn) from platform 5 towards Rheinbach / Euskirchen / Bad Münstereifel. Your stop is Rheinbach.
From Cologne central station, take the DB Regional Express towards Bonn, leaving the train at Bonn central station (Hauptbahnhof). From here proceed as described above.
Alternatively, take the train from Cologne central station towards Euskirchen. Change at Euskirchen to the DB Regionalbahn towards Bonn. Leave the train at Rheinbach.
In Rheinbach, Keramikerstrasse runs parallel to the railway line on the side opposite the station buildings, before turning to the left away from the tracks. It leads almost directly to the university, taking about three minutes on foot.