Gather up lots of pencils. Be sure you have a time and a place scheduled, and have someone in charge of getting the fire lit, and announcing that it's time to gather. The Theory One entrant might win more than one category. One piece can be judged best in more than one category. Entrants don't need to know what they're doing until just before the contest.
Entrants can join late without ruining anything.
Judges don't need to know if they can judge until just before the contest. They do need to be able to stay for the whole contest, but if someone has to leave early, he can just destroy his ballot and it's no problem. You can have as many judges as you have ballots and paper for, or as few as one. The more judges you have, the fairer the choices should be, but don't worry if you only have a few judges. If you only have two or four, though, either give two prizes in case of tie, or decide a way to have a tie-breaker.