If you decide to take a subway, you will need to take the Airport line.
And then you should transfer at Gongdeok station line6 and get off at Anam station.
After you come up Anam station exit 3, turn around and you will notice an intersection.
At the intersection make a left turn, and come down the street about 1minute.
You should be able to see "Anam Hostel" on your right.
We strongly recommend you to use this route. It will take about 90 mins.