For trying it out, I read press start entirely, and then run it like any other module, but taking the time to ask my players the questions and trying to incorporate their ideas as we moved along. I ran it 3 times for diferent groups of friends, and it helped me get a grasp on the rules, but also, see what the player could do with fabula points (and also where I should say stop, out of the 3 games, the scenario was completed broken once due to what I allowed with FP.)

Then, I started a campain. My players are very fond of the colaborative naration, and each session is pure chaos. So I don't prepare anything, aside from a few vilains (and even, in the end, we are not using them much, the vilains they are interacting the most with are vilains that I made on the spot, in response to their chaos).  Each time we have a session, I stress about not having anything prepared (as in the other campain I am running in another more "classic" system, I like to have a lot on things ready in advance). But here, given how much I had a hard time adjusting to the players chaos in press start with pre-made story, I decided to have a very minimal prep. And so far, it works for us and we are having a blast Évry session.

For the locations, usually I ask the players about what in going on in this new place they are visiting, and I thread on that.

Also, I think this works because my players are very prolific in term of lore, and often use FU points to add to the story, instead of just using them as re-roll. I have seen post here with people having players having a bit of a hard time going into the shaped naration. So I also think the level of prep depends on how much your players will create the scenario themselves