To become an ESL teacher in Korea, you must meet the criteria given below:

English must be your native language.
You must hold a Bachelors degree or higher from an accredited university.
Required documents  
Bachelors degree or higher from an
 accredited university

Officially sealed university transcripts
  with an ink stamp/sticker over the back flap
Passport with at least a year or more of
  validation left

4 passport size color photos

Police Clearance certified by an apostille or
  Korean Consulate/Embassy