KE uses the main terminal while Delta uses the Satellite terminal at ICN.
It's an extremely easy transfer, but once you take the underground train to the satellite concourse, there is no way back (aside from being escorted
).
Upon arrival follow the signs to transit. Clear transit security. You'll be on the departures floor in the main terminal. KE lounge is here, don't know about the access policy.
There are free guided tours into the surrounding area if you'd like to get out of the airport. Shortest trip takes one hour, and I've done this on longer layovers.
Anyways - from the departure level in the main terminal where KE is, just follow the signs to reach the satellite concourse.
Connection is easy peasy and it's a great airport.
SDF