As a fellow judge, I recommend that you keep an eye on the "recent events added" tab on the website. The day new contests are added you want to apply if you're interested in that contest. Some contest organizers select judges on a first come/first serve basis, some try to mix the field with master judges and less experienced judges and some use their previous judge pool and fill in the open slots. You just want to get your application in as soon as possible. If you apply early and then learn that you can't judge a contest make sure you let the organizer know as early as possible as well. You don't want to get a reputation of cancelling at the last minute.