Many newcomers to organized play campaigns envision the preparation of an adventure as that of the old GMs of old, who prepared everything, had map completely drawn, had the perfect minis for everything (which they painted, back in the day), and a super-awesome storyline to go with it.
One of my teachers back in College kept shocking us because he would walk into a class flipping a small chalk in his hand and nothing else. He "taught" computer architecture and informational theory (the quotation marks are to present the implication of teaching... the books were much more useful than listening to his "planned lesson"). For some reason we all had the impression that a teacher had to have a briefcase, a big stack of papers, and a coffee cup. He had nothing of that. He was a lousy teacher, very, very lousy. I checked his RateMyProfessor.com rating and he is indeed rated extremely low (1.3 out of 5 is his best score), can’t say I disagree.
Why talk about such a bad teacher in an article about how to prepare to be a good GM? (I consider myself to be a good GM). The man knew his stuff, he couldn’t communicate it to save his life, but he knew his information theory.
0 - You WILL be wrong
Like any married guy will tell you, you will be wrong. It’s okay. Don’t dwell on it. It’s okay to make mistakes, if pointed to a mistake you made, retract, and keep the game going. I have a saying that “nobody cares if nobody dies”. That’s mostly true. If a call you made was wrong and the player shows you, be graceful.
It’s OKAY to be wrong.
It’s NOT OKAY to be defensive or unmovable about it.
It’s NOT OKAY to constantly talk about that judgment call.
1 - Know YOUR stuff
I’m sure I don’t have to tell you, but start by reading the adventure background and summary. Pathfinder Society and most other OPs do a good job with those. After reading those two sections you know 1- how things got to that point 2- what the PCs are expected to do.
In preparing to run an organized play event, you need to know your stuff. I mean know YOUR stuff. That means understanding what the PCs will be trying to do and rolling with the punches. As a GM, you don’t need to know all of the intricacies of classes you never looked at unless you have to run one.
For example, I still have a rudimentary understanding of the following classes (mostly because I never cared to play one yet): Alchemist, Druid, Magus, and Summoner. I know enough to run them as a GM, but could not come up with one on the fly. If a player shows up with such a character, I can run them through an adventure without difficulty, but the many options... Can’t say I read too much about them. I can always ask the player to show me a rule.
Make sure you know the options your villains have and those are all you need to know. Keep the tactics in mind, but don’t stress about changing them.
2 - Do not memorize everything
Really. You don’t. Understand what's going on. Understanding and memorizing are completely different things. I read it once, pay attention on more complex scenes, and give a cursory glance to the villains's stat block. Don’t think about the stat block. Read the adventure like a novel, without worrying about the game math.
Get a feel for the flow of the adventure, note elements to research ahead of time, rules you need to review.
3 - Don’t waste your time worrying about the PCs
Too many GMs, especially those who do not GM regularly worry about the PCs and what they will do or how they will throw curve balls at them, and try to plan every way to handle what they will do.
You cannot predict who will play, what character they have, how they will react to a situation, how their group dynamic could influence a given scene, how the dice will roll (or not roll) for you or the players. Too many GMs worry things they can’t control that they become lousy at it. YOUR part is to know the adventure. Point A to Point B. The PCs WILL go off-script. That’s actually one of the times for you to shine and make the adventure your own. Don’t panic about it before the game.
4 - Prepare the highest level and scale down
I told you before to spend little time on the stat blocks. After you understand what the goals and story elements of the adventure is, Now is the time to go back and read over the stat blocks. Pay attention to the villains’ feats, tactics, special attacks and abilities.
By preparing for higher levels, scaling down is MUCH easier than on-the-fly researching a villain and all his weird abilities at a higher level than . We all know that higher level baddies have more cooler, shinier toys than lower-level ones. The exact values and numbers don’t matter too much (again understand not memorize). I rarely look at their AC, Saves, normal attacks or hp totals before the game. Those values are easy to find when players are in front of me.
5 - Every opportunity is an opportunity to learn about the game
So now you should have 2 lists of elements to look at. First a list of rules important to the adventure. Second some monster-specific rules (or even monster themselves).
Before the game, look up elements you know the villains are likely to do: sundering, trip attacks, underwater combat and the like. Read the entries all the way to the end.
If you see another spell/ feat/ ability/ glossary element that is vaguely related, read through it. When the players are sitting in front of you is NOT the time to do that. That way you gather the knowledge and over time, you will remember reading about this or that. Your brain’s knowledge centers will be focused on elements found in the adventure.