Here's Fulbright's manual for teaching assistants. Although a little outdated, it has some very useful content: Korean language (for the classroom); sample lessons across the four skills (listening, reading, speaking, and writing); ideas for holiday and travel lessons; and ideas for teacher workshops.
In addition, I've also attached EPIK's guidebook for GETs (from
http://epik.go.kr/ under What's New, no. 137). In two parts, the first section is "A Guide to Korea" and the second is "Things to Know as a Guest English Teacher (GET)".
