I'm not directly involved with the New Gamemaster Month program, but I thought it'd be fun to kibbitz and offer a little extra advice. The system is Trail of Cthulhu, and the sample adventure on offer is Midnight Sub Rosa, in which a group of investigators are sent to Rosa, Alabama to recover a diary written by necromancer Ezekiel de la Poer. If you want to download the scenario, I recommend you wander over to the New Gamemaster Month website where there is a download link.
I'm going to start the discussion with a note on historical accuracy. Thursday's New Gamemaster post says that you "don't have to have every nook and cranny of the setting committed to memory, either - in fact, the setting is yours to craft, and elements you interpret differently than what's in print (on purpose or by accident) make the setting your own. That's a feature, not a bug." This is absolutely true, but some people may find it difficult to believe, because of the historical accuracy problem.
Trail, like Call of Cthulhu and many similar titles, is set in a particular time period. This can cause novice Keepers, and players, concern. Sometimes this is because certain aspects of history are, at best, unsavory. The example scenario spends some time talking about race, a topic that's bound to come up in 1930s Alabama. However it just as often causes problems because people don't know enough history, and feel the lack. They get nervous that they're "doing it wrong," or worry that a particular technology might not have been available at the time. What does it mean for the scenario when a player says, "I turn on my flashlight," and someone else at the table says, "did they have flashlights back then?"
First thing: don't panic. Confidence, as the Thursday update says, is the only secret sauce. If you, as Keeper, choose to rule thus-and-so, it doesn't matter if history contradicts you.
Second thing: a little history can be very useful, and history's easily had.
You don't have to bury yourself in textbooks. The game manual is your first stop, but there are other sources. Writer's Guides for pretty much every period you care to name are available at very reasonable rates, and because they're pitched to people in exactly your position - creators seeking background knowledge - they're very readable. I have a copy of the Writer's Guide to Everyday Life from Prohibition to World War II, which I see is going for silly money on Amazon, and while I wouldn't base my PhD on McCutcheon's work, it's certainly good enough for game night. It doesn't just give you timelines and dry facts; it has a list of slang terms, an essay on crime and a selection of cop slang, bits on transport, clothing, radio, music, dance, what people were reading, watching, talking about. If ever you want to add color to a scene, this is the kind of detail you need.
For example: Death Valley Days is a radio show that started in 1930 and went on, in one form or another, until 1975. Some episodes are available online. Nothing could be easier than to have that playing in the background; there are many apps that play old time radio, most of them for cheap or for nothing. Heck, even if you don't use these old shows as background noise, it's still worth listening to a couple, if only to steal characters to use as NPCs.
However there's another way history can help: it can give the Keeper ideas. Lots and lots of juicy ideas, many of which can be data mined from Wikipedia.
Consider the telephone. By the 1930s they were common; Rosa, Alabama has a party line, according to the scene Exploring Rosa. The scenario notes that the house where most of the action takes place doesn't have much use for electricity, but there are telephone poles. The characters are presumably staying at the lodging house, which definitely has a telephone.
Let's take a step back. What exactly is a telephone exchange? Well, when telephones were first used, they were single-function devices. You had a phone, it was connected by wire to another phone, and that was that. You could only call that one phone. If you wanted to make calls to other people, you had to install new telephones. This wasn't particularly useful, so someone came up with the idea of the exchange. Everyone's phone was connected to the exchange, and when you wanted to call someone you contacted the exchange and told them which telephone you wanted to connect to. The operator then physically connected you with that phone. This system continued, in one form or another, until the 1960s, when automation replaced human operators. If you read old authors, like Agatha Christie, Dashiell Hammett, or Dorothy Sayers, you'll notice that whenever a character picks up a phone they talk to the operator first, and say "connect me to [letter code] [number]," and after maybe a few seconds, there's a connection. The letter code identifies the exchange, the number, the telephone. Usually the number is anywhere from three to five digits long, depending on the likely number of subscribers. The smaller the catchment area, the smaller the number. The letter code is often turned into a word for easy memorization, as for example with Susquehanna 4 7568.
That tells you, as Keeper, three things. First, that connecting with another phone is a lengthy process. You might have to wait a long time, perhaps several minutes, before you finally got to speak with the person on the other end. Second, that there is a human being, the exchange operator, between you and the other person - and the exchange operator can hear everything you say.
Third, that the number of numbers is limited. Take a look at Susquehanna 4 7568, an episode of The Naked City, a TV show that aired in the 1950s and early 1960s. The story kicks off when a young woman, new to NYC, gets a phone installed at her flat, only to discover that her number used to belong to someone else and she's getting his calls. Exchanges can only accommodate so many subscribers; eventually they have to re-use numbers. In The Naked City, the woman overhears a murder. In Cthulhu, the investigator might overhear almost anything.
The Keeper can complicate this further. As Rosa is a rural community, it has a party line. These are cheaper to run, and don't need an operator. In broad terms, everyone is connected to everyone else, on a loop system. The obvious problem being that there is no privacy on the party line; everyone gets to listen in, not just one operator. Moreover a user can monopolize the line, preventing anyone else from making calls, and this can happen by accident, when someone doesn't properly disconnect after a call. Party lines were still a thing even as late as the 1980s; it wasn't until people started using phone lines for other things, like answering machines and computer modems, that they finally died out. Stephen King, for example, references party lines in his fiction more than once.
The scenario says that the party line only connects three locations, but as Keeper you should feel free to modify a scenario to suit your needs. Remember, it's a feature, not a bug. If you want that party line to connect to other places, you can. Probably not very many other places, since, as written, Rosa is dirt poor. However a couple of the outlying farms could also have telephones, and there's no reason the Derby House shouldn't also be on the party line. It may even be that someone's illicitly connected; after all, it's not as if anyone's marching up and down the line looking for extra connections. This may come in handy if the Keeper intends to run any of the additional material provided with the scenario.
There are two plot-related points to make here.
First, the party line is an excellent way to provide clues, particularly via Oral History. People talk. Often they can't help themselves; they give away secrets and gossip in equal measures. Just listening in can provide an investigator with all kinds of dirt. Imagine what might happen if the investigators catch Sheriff Barnes on the line, discussing the situation with the state police, or listen to some of the citizens of Rosa dishing dirt on people's families and less-than-reputable pasts. "Of course, nothing good ever came from that side of the family. Why, my dear, didn't you know ..."
Second, if the investigators use the telephone, they have no way to ensure their conversation is confidential. It's wiser to assume someone's listening. Or something. After all, there's that o-so-intriguing section about Ghoul Changelings; imagine what might happen if one of those was listening in on an investigator's call for help.
But perhaps the most significant non-plot-related point to make is that a party line can add a lot of color to an otherwise drab setting. Gizmodo makes a similar point, when talking about barbed wire lines. People used to live their lives on these connections. They'd play music, talk about local politics or sports, read newspapers to each other, recite the weather report, pass on important news or alerts to the group. In many ways the phone lines acted in the same way a forum post does today; it passes on group messages, and alerts the group to important information. It adds that extra bit of vibrancy to a location if the Keeper bothers to add a few bits on non-plot related bits to a party line call. "Missus Sullivan's dog's missing? Better let the boys know, they might see 'er out in the long pasture. Gantry's sow dropped a litter of six last night, says he'll be selling off the weaners in a week or so, you want in on this you better go see old man Gantry afore they all leave the nest..."
So, going back to the original point: just by knowing a bit of history, in this case about the telephone, the Keeper can insert clues, link to scenes (even Antagonist reaction scenes), and provide enough color to make the setting come alive. None of this requires specialist knowledge; you can data mine from Wikipedia and similar sources to get everything you need.
So why the telephone? Why not? Everything has a history. Cars, nightclubs, public transport, trains, newspapers, factories and diners and a thousand things besides. You don't need to be an expert. You just need to ask yourself, "is there anything useful here?" Nine times out of ten, the answer's yes. Seek it out, and use it.
To all new Keepers and Game Masters, welcome to the hobby! I hope you enjoy yourselves.