To get from Incheon International Airport to Yonsei University in Seoul by subway, you can follow these steps:

1. Arrival at Incheon Airport: After arriving at Incheon International Airport, follow the signs to the Airport Railroad (AREX) station. You can find it in the airport's transportation center.
2. Purchase a Ticket: Buy a one-way ticket for the Airport Railroad (AREX) Line. The fare may vary depending on your destination, but you'll be heading towards Hongik University Station.
3. Board the Airport Railroad (AREX) Express Train: The Airport Railroad (AREX) offers two types of trains, Express and All Stop. You can take the Express train, which is faster and more convenient. Make sure you are on the train heading towards Hongik University Station.
4. Travel to Hongik University Station: The AREX Express train will take you directly to Hongik University Station. The journey takes approximately 1 hour, depending on the airport terminal you arrive at (Terminal 1 or Terminal 2).
5. Transfer to Line 2 (Green Line): At Hongik University Station, you will need to transfer to Line 2 of the Subway, which is also known as the Green Line.
6. Ride Line 2 (Green Line): Board Line 2 (Green Line) heading towards Sinchon Station.
7. Arrive at Sinchon Station: Get off the subway at Sinchon Station.
8. Walk to Yonsei University: From Sinchon Station, you can walk to Yonsei University's main campus. The University is within walking distance from the station, and you can follow local signs or map app.