youd pick a favorite team, bet fake money on the outcomes of games, then spend the fake money at the end of the week on votes to determine major events affecting the league as a whole (including rule changes) and your favorite team/its players.
For example the big vote after the very first week was Open The Forbidden Book vs Dont Open The Forbidden Book. The book was opened. The next week, the umpires in games suddenly had a very small chance of incinerating a player on the field, which killed them and replaced them with a new player with new stats. Strange weather phenomena like solar eclipses, peanuts falling from the sky, etc became a fixture of the game as well. Fans created a lot of backstories for players and stuff like that, creativity from the fans was often incorporated into the actual game.