When you're ready to choose a character, read the rules, and are very sure you want to stick around for the best and worst, fill out an application. Please be detailed. And no choosing of already taken characters. You should hear from Koh in 7 days or less. Thanks.
When you're done typing your application, you have to mail it to Koh, using the subject "Suikoden 2 RPG application" or something similar. If Koh asks for additional informations or clarifications, you have a week to reply until Koh decides to trash your application, so please be hasty!
Read the FAQ. Read it all. Make absolutely sure that you both read and compare and contrast the examples of accepted and rejected applications in particular. If you put in a one-line answer for Character History, Future Plans, or Personality, you will be rejected.
That's important, so Koh will repeat it. IF YOU PUT IN A ONE-LINE ANSWER ANSWER FOR CHARACTER HISTORY, FUTURE PLANS, OR PERSONALITY, YOU WILL BE REJECTED. Don't even try. Koh also likes it if you display knowledge of what your character's previous story involvement has been. So, if your character has existing chapters, read them and include a summation of those chapters in the Character History section. If your application fails to demonstrate that you're familiar with the FAQ, Koh will deny it. Koh is one tough cookie.
And another note (geez, this should be in the FAQ, probably, but no one seems to read that -_-). All applications for original, taken (which are listed as taken on the page), or excessively out-of-character characters will be automatically thrown out.