Heinz Y.
Google
This review is for all the people who need to use AREX from Seoul station, which is problematic if people did not use this coming from the airport.
When you exit the Metro station, you must leave and go outside. There, you will see escalators taking you to the KTX concourse where you buy your train tickets. This is a major hub. You will see numerous KTX train ticket booths.
You want to go into the concourse of the train station. You should see to your right a white information booth.
At the end of the hall to the far left, you will see a yellow/orange arrow indicating Airport Express (AREX). Go to that direction, go down the escalators to B2.
At the B2 level, go left, you should see an orange information booth. To the left of the orange both is the entry into the AREX platform station. You will see the kiosks. If you bought tickets in advance and have a QR code, the scanner at the gate can do it.
I pre-scouted the area, and don't forget, you still need to get to the Train Platform. So go go go, as it will take time to get to your train on time.