Terminal 1 in CGN has now one central arrival area. (basically after luggage belts, everybody appear out of the same exit) From there walk straight towards Terminal 2 and make your way all the way downstairs where the train station is.

If you travel towards the south from the airport, your train will likely depart from the closer platform, if you go towards the north (through Cologne city), your train will likely depart on the second platform.