I've been running RPGs for over forty years. (Eek.) By now I have a
reasonable idea of what my notes before a game should look like.
I use the same general format whether it's a one-shot or a
session in an ongoing campaign, though the weights of different things
change. (I've written adventures for publication a couple of times,
but both of them have been constrained by a specific house format.)
It's a counsel of perfection of course. Sometimes things just get
jumbled in wherever they seem to fit, especially on a first pass.
Introduction
This bit is minimal during an ongoing campaign, but for a one-shot
it's very important: not just the basic scene-setting, but what things
need to be emphasised at the start of play. This is, broadly, the
stuff the PCs get dumped on them at the start, before they begin to
make decisions.
In a scenario I'm going to be running this multiple times, this tends
to accumulate key things I should mention up front to establish the
right mood.
What's Really Happening
What does the initial mystery mean, and what is the overall plot? This
will refer to many things that come later; it's a quick summary for
orientation.
(When this is missing from someone else's scenario, I find it hard
work to read because I have no framework into which I can fit things.)
Events
What will happen if the PCs don't muck about with it? Some things
might be on a timetable, some might be when a particular person is
unobserved, etc. In turn, the consequences of these things get listed,
especially ones that the PCs will notice.
(And there must be a timeline of what's already happened, if that's
more than trivial. Not only is this helpful in general, it's good for
double-checking that I don't have things happening before the events
that caused them.)
Places
A section for each major location where PCs can do things. (That might
be rooms of the house, places in the town where they can have
encounters or find clues, etc.) Also how to get between them and
travel times, if it's that sort of scenario.
People
Who are the NPCs, what do they want, how do they talk?
Pretty much everything fits into that, for me. I rarely use monsters,
but I suppose they could go after people if they were needed.