Well, the first thing to do is call or check the websites of local haunts and see if they want actors. Get into contact with them and ask how you can sign up.
As for me, I read an article in the newspaper about Dr. Pain's stating they're always open those who want to volunteer. I drove down to Pain's soon thereafter, met a few people, filled out some forms (no interview), went on a tour, got the info on who I was going to be, what I am to do, and that was it.