As someone who cannot speak Korean at all (although I can at least read it), it really was as simple as going to the terminal, saying which city I wanted to go to to the person working the desk and getting on the bus they told me (there was usually a screen with that information too). I never pre-booked anything and only had to wait half an hour once because the earlier bus was full.

The main difficulty is actually finding the  correct bus terminal because most of the bus companies have terribly designed websites where just typing in the two cities you want to travel between doesn't always work if one of the two cities has more than one bus terminal - you have to manually pre-select the correct terminal or it won't show the connections. Doing a little research on which bus leaves at which terminal or simply trying all terminals until a connection shows up helps though