I'm the Forever GM. I've been GMing games since starting with D&D (white box edition, heavily homebrewed) since the mid-1970s. We did dungeon crawls and were murderhobos. During that time, we tried d6 Star Wars, Paranoia, James Bond, Space 1889, Shadowrun, Stalking the Night Fantastic, etc. It was only after college that I got into more of the storytelling aspects of roleplaying (by playing Vampire and GURPS). I only started to intensively run CoC in 2011. My gaming style has evolved from combat based (random combat encounters) to more story based (coming up with clue trails, clue details, NPC motivations, plot twists, etc. on the fly).
Anyway, I'm seeing a number of people who've never GMed asking how to prep for a game.
Here are my steps (do steps 1 to 6 days earlier and those steps takes about 4+ hours):
Read it quickly (you can skim or speed read it) just to understand what's going on, to understand the story. Most CoC scenarios has a What's Going On section (Keeper Info, Overview, Background) at the beginning of the scenario spelling out exactly what has happened. Don't worry about stats, mechanics, or spells, just read it for the plot. The reason you need to know the plot is because this lets you improvise clues and clue trails that makes sense during play. If you don't know the plot, then you might create conflicting clues that'll confuse your Players.
Cut-n-paste handouts (either copier for in-person, or screen shots if running online). Read them also because they sometimes contain info not in the scenario text. I've been blindsided a few times when a Player asked about something that was in a handout I didn't know about. In most published scenarios, copies of the handouts are also at the end of the scenario or in the appendix in the back of the book.
Google pictures for additional handouts. Note what is missing in above step. You'll generally notice you're missing a NPC portrait, a location map, picture of a landmark, or period objects that you're not familiar with.
Have pre-gens (pre-generated PCs) ready. I don't recommend having Players make PCs in their first game. Just provide more pre-gens than Players, so at least the last Player isn't left with no choices. Also maybe gift or reserve 100 skill pts that they can add to any skills they want (other than Cthulhu Mythos). An extra PC can also be used as a replacement PC if a PC dies early, unexpectedly; it's no fun to be at a convention and your PC dies 1 hour into the game and the GM tells you you can't play anymore. When I run high lethality games, especially at a convention, some systems (Godlike, Mörk Borg, Alien) are more lethal than others, I make sure there are plenty of backup PCs. If they are creating their own PCs, have them use www.dholeshouse.org. Some games, I use a spreadsheet when I create characters to make sure all skills needed for the scenario are covered and the PCs are balanced.
Highlight a copy the scenario (hardcopy or digital, I hate writing in books). Read the scenario again carefully and use a highlighter on important bits. You can use different colors for GM info, stuff to read aloud, save throws, names, important aspects, etc. Create more handouts that you've missed on the first pass. This is also when you'll realize that parts of the scenario either doesn't make sense or is missing details and you'll have to modify the scenario or add additional details that fleshes it out.
Create a single page bulleted outline of important stuff that must happen. Also add your own notes.
Right before the game, I reread again to refresh my memory and add any additional highlights or handouts I missed. At a convention, I generally don't look at the scenario again (unless I have to), but use the single page bullet pointed outline.
Have pencils, dice, character portraits, character sheets, maps, handouts ready for running. I don't like using a GM screen because it's a barrier between me and the Players. I also roll dice out in the open and don't fudge rolls, but I might fudge an opponent's HPs or skills in the Players' favor. If I have to hide info, I use a clipboard and make sure I have a cover sheet hiding the information. If I need a rulebook for various charts or tables, I use a sticky note as a bookmark for that page or a cropped screen shot if I'm running online. At conventions, I arrive early to pick my spot for all my stuff and rearrange the chairs if necessary. I then layout all my stuff.
At conventions, where timing is important, I use a watch I can casually glance at to see what time it is and use it to compare to where the PCs are in the scenario, so I know whether to speed it up or slow down. I also have optional scenes I can throw in to lengthen the scenario if I'm running too fast, or know which scenes to drop if I'm running slow. I also don't unnecessarily lengthen a scenario, I'd rather end early than run out of time. I've been in games where the GM ran out of time and had to summarize the ending. The best part of most movies is the ending. How would you feel if instead of the climatic ending of a movie, you get a screen crawl summarizing the ending?
Call for 10 min breaks when you're stuck, so you can think about stuff and for people to use the bathroom or get food. Do this a few hours in or at a natural stopping point. Most convention games have at least one break after 2 or 3 hours.
For convention play or a one-shot (where PCs don't come back), I always allow the Players to do an epilogue for their PCs, a short sentence or paragraph as to what happens to their PC after the scenario ends and how the events have affected them or their lives. This gives some closure for the PC.
If you have to do more than one session because you didn't finish, try to stop at either a natural stopping point (after a reveal or end of combat) or a cliff hanger. Then schedule a follow-up session.
You can ask for feedback on what the Players liked or disliked and what you can do better (this is more for games at home or with your own group, not at a convention). At a convention, there might be feedback forms that are handed into the convention runners, not to the GM. Comments might be forwarded anonymously to the GM.
Pack up your stuff and leave the convention room as clean or cleaner than when you arrived.
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