Well, think of it just like buying a game you've never played. Most of the mods include the rule book inside the mod itself. But you can also look up the rule book online. It's also recommended to watch the multitude of Youtube videos out there about how to play these games. And really, there's at least one of those videos for every game. I've yet to see any game that doesn't have it. Here are a few screenshots of my workshop and a shot of what Gloomhaven looks like:
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