I did find a book but it's very complicated. There are single-elimination tournaments where if you lose one game you're out, and there are double-elimination where you have to lose two, and then there's something called Swiss Style that pairs people up based on whether they're brilliant at it or pants at it, and in a round-robin tourney everyone gets to play everyone else and whoever wins the most games is the champion.

I think round-robin would be best but I'm afraid there isn't time, not in one afternoon.

If Professor Vector wanted to help it would be brilliant. I saw you ask Draco about the book, do you want me to send Harry's mudblood over with it? He could bring it to wherever you want him to meet you.