theatreofcomets says:
... are we all deciding on a shared game first ...
I don't think so. This is (as I currently see it) for all types of PbtA, so the games/scenes will run the gamut of genres and styles. We can always spin off a game from scenes that look interesting here.
Just tell us enough to get us started in the scene.
theatreofcomets says:
... presumably after creating characters first ...
Depending on the game we use, we might be able to get away with not creating characters. The characters (or character stubs) we use in one scene may not be applicable to another.
If we want to do full on character creation for one of them --as an exercise-- we can do that. If we need full characters for the GM to envision what is happening in the world, we can do that.
Not all players need participate in all scenes, of course.
theatreofcomets says:
... I am happy to go away and read DW first if that's the best course of action.
Not necessary. That was a suggestion for another player.
If you have another system that you have read, and want try running a scene or scenario from that, go for it. If it is something the rest of us have not read we might not be able to give you specific advice, but most general PbtA advice should still apply.
If we have not read it and you have to
teach it to us... believe me, teaching is the best way to learn. You may get it wrong the first few times, but having to go back and reread enough to understand enough to be able to explain it to someone else who is struggling to understand really gets the details in there.
The GM generally needs to know a lot more (about the rules and about the world) than the players ever do. It can, at first, feel disappointing to not get to tell the players half of what we had prepared, but having it ready for if it comes up can be reassuring.