Its pretty simple, just follow the instructions to install GGPO:
http://ggpo.net/download/
Then download the ROM for KOF98 and place it in the \GGPO\ROMS folder. (you can find torrent for GGPO roms easily)
Launch GGPO, create an account and login...You'll see a chat like window with a bunch of rooms for different games...click on a room, and you can challenge whoever is free in the room.