Note that you must be flying Korean Air, Asiana Airlines, Jeju Air or China Southern Airlines to check in at the City Airport Terminal. Also, you have to check in at CAT at least three hours before your flight is scheduled to depart.
Purchase a ticket for the Airport Railroad Express Train (AREX) (Tickets are ₩8,000, adults; ₩6,900, children). You can do so either at the counter or an electronic kiosk on the B2 floor of Airport Railroad Seoul Station. Check the time table here.
Walk a few steps over to your designated check in counter and present your flight information, passport and AREX train ticket. You’ll be checked in just as you would at the airport, and will be presented with your boarding pass. Drop off any checked luggage… they’ll deliver it all the way to Incheon so you don’t have to!
Wait 10 minutes for your luggage to be cleared. (Make sure you don’t have any lighters or batteries in your bag!)
If you’re not called, proceed to the immigration office next to the check in counters and have your passport stamped. You can instead go through the regular security and immigration lines at the airport, if you chose to do so, but it’s preferable to do it here.
Take the elevator down to the platform on B7 and board the train. (Check your seat assignment on your ticket receipt.) Enjoy the 43 minute ride!
Upon arriving at Incheon Airport, go to the Departure Hall on the third floor. You can use the designated entrance to be fast tracked through security and immigration.