There are no rules, so, really whatever you want. At minimum there’s usually (but doesn’t have to be) a theme of some sort, maybe some rules/restrictions (but doesn’t have to be), a deadline, voting or random selection of top/included tracks, prizes (but doesn’t have to be), release via SoundCloud playlist
Here’s an example of the first one I organized a while back, but there are many examples/ways of organizing the thing. Really, it’s whatever the (dis)organizer wants.