The first 2 hours after landing are the most important.
Don't rely on airport Wi-Fi. Having mobile data is crucial for using navigation and translation apps as soon as you step out of the terminal.
Decide whether you need a bus or a train. The Airport Limousine Bus is usually the best choice for travelers with heavy luggage or those staying near major hotels.
Google Maps has limited functionality in Korea. Make sure to download Naver Map or KakaoMap for accurate walking and transit directions.