D&D 5E DM Prepared one page of notes.

[Fralex]

Maybe he spent 45 minutes writing up the sheet, but probably hours and hours thinking about the mood he wanted to create, how to structure the encounters, the plot, the NPC personalities, and so forth.
I also often only have a page or two of notes, but they're there to reference the bits in my head. It takes me usually a couple of weeks to think up a scenario in enough detail that I feel ready to run it.
In fact, I find it MORE difficult to run an adventure from a published module because all the interconnections between NPCs and monsters and encounters are not in my head but stuck on the paper.

This, speaking from personal experience and things I've heard other very good DMs say, is exactly right. It can totally work to run an entire campaign with just a few scraps of paper covered in random scribblings about ideas you have, but don't think for a second you can just make something up as you go without spending a ton of time fleshing out places and characters you plan to use first. You don't set in stone the way everything fits together, but the pieces need to be developed enough that when you see the right opportunity you can just weave one into the story. It's a little harder than it looks from the outside.

That said, it is a FANTASTIC way to DM. You're never worried the story is getting off-track because you can always gently guide it towards one of your pre-made plot points or something you can use will come up on its own. And if you're worried you won't be good at this sort of improvisation because you were always bad at those improv games your drama-inclined friends liked to play, well, I'm no good at those either and can still pull this off. I recommend this campaign journal for inspirational reading (it's also just a really fun collection of stories, so your time won't be wasted whatever you learn or don't learn).

 

[The Grassy Gnoll]

The best game session I ever played in was a chase/investigation adventure playing Expert set D&D, where my character (can't recall who it was, now, after 25 years+) spent the whole session unravelling an intrigue around the town of Threshold. Absolute blast and afterwards my DM admitted he'd made the whole thing up on the spot. I was so impressed!

 

[chaochou]

I really want to adopt this style of DMing. It's amazing to me.

Yeah, great when it comes together. The good news is that with a bit of practice and a smattering of confidence it comes together surprisingly often!

I would say that if you want to learn to GM like this, there are games out there written specifically for it and which explicitly guide you through how to do it in the game text. Apocalypse World, Dungeon World, some of the FATE games. I very much doubt your one-shot GM runs just D&D or learnt these techniques through D&D.

I've been running FATE and now Apocalypse World. My current game of AW I've run five sessions, each of four to six hours, with less than a page of notes in total - usually nothing at all. And it is stupidly good fun for all of us. The game codifies what you need to do to run it - and prep isn't one of those things. It's more about trusting yourself and your players and having faith in your own imagination. Once you've run a game like that there are a lot of transferable ideas and skills into other games.

Skills and techniqes take you so far and the rest is down to the players and whether there is any mojo around at the time. Great games are a group responsibility - its always the collective contribution between me and my friends which creates anything beyond 'decent'. Player buy-in and willingness to act are critical and when I'm in the zone I find improvising is less about 'running' a game and more about 'allowing it to happen'.

Best of luck with it.

 

[Schmoe]

It's really not my style to just go in with minimal prepared material. I find I can craft much more compelling scenarios and scenes when I have time to think them through. If I'm running homebrew material, preparation for an adventure goes something like this:

Develop an area (I'm always doing this)
Develop NPCs (I'm always doing this)
Flesh out likely developments in the plot the group has been following
If those developments seem like they are leading to a site, flesh out the site
Come up with a few updates for character-specific plots

When we sit down to play I try to be very reactive and let the game go where the players want it to, but I wouldn't be able to react in a compelling way without all of the preparation.

I'll give an example from one campaign long ago. By the end of a session, the party had decided to seek out some information from a sage who lived in a forest about a 2 week's journey from the town. I spent the week developing an adventure around the tower that involved a goblin druid, his corrupted brother, a rift to the Shadowlands, and slowly spreading corruption in the forest. The adventure included the druid's tower, shadow-templated creatures (this was 3e), and a cave complex. There's no way I could have come up with all of that or even most of that "on the fly". Taking the time to develop it ahead of time allowed me to include subtle details that tied in with long-term plot arcs, details that I just wouldn't have thought of in the moment.

I think it's amazing that some people are able to use so little preparation and run a great game. However, I know that I'm not one, so I'm going to stick with what works for me.

 

[Blue]

Most excellent. I run sandboxy with lots of plots thrown at the wall that the characters can choose to follow or not. I find that if I plan a lot on concept ahead of time, just a little bit of notes on options and things from previous sessions can let me improvise no matter which way the party goes. Often a session will start that I have no idea which of several plots they will follow up on, and often they will come up with an option I never saw coming.

That said, easy printing has made me lazy. I've got a cheatsheet with pertinent plot info about characters that I keep for every session, reminders of pipe I want to lay and things that might come back from other sessions. Notes on NPCs. Oh yeah, and some stuff about this session. Then handouts, any new magic items (I hand out cards), and monster stats.

Also have a document called "checkovs_armory" (a play off Checkov's Gun) with all sorts of open plot threads and NPCs that I can have reoccur or influence the session that I add to after a session and browse every couple to see if there is anything I can have boomerang.

 

[GameOgre]

I remember back in my AD&D days long ago I used to run adventures on the fly with nothing but my DM notebook but those days were decades ago. Somewhere along the way I got it in my head that I needed to stack my prep work sky high and devote more time to prep than actual playing.

I'm determined to get my low/no prep mojo back after playing in that game.

 

[Fanaelialae]

The best DM in our group has always run this way. Although instead of a page of notes, he draws a one sheet map with virtually illegible scribblings on it to jog his memory. I've collected his notes after the end of each campaign for years now, trying to decipher the madness to his method. He's told me that he spends a lot of time during the week thinking about game, but it usually only takes him about 30 - 60 minutes to actually put his "notes" in order.

I tried to mimic his style for years, unsuccessfully, but with 5e it suddenly "clicked" for some reason. Last session I ran an impromptu game (one of the players couldn't make it last minute, and "best DM" didn't want to run without him) using notes I had left over from the previous session I'd run, and I was able to run a great 8 hour session, despite having already run an 8 hour game from those notes in the previous session. I was really surprised it worked out as well as it did, because they'd actually finished the "adventure" in the previous session, and this session was mostly just tying up a few lose ends and interacting with NPCs.

The Lazy Dungeon Master, by Michael Shea, has a lot of great advice for prepping this style of game (though being a VERY lazy DM, I have yet to actually finish reading it).

 

[drjones]

I enjoy the writing process and because of the time constraints of old people even though I DM with two groups I get more time to write than to play. So I don't mind a little prep but as I have gotten into creating my first 5e campaign using the DMG and MM I have found that I am spending all my writing time on ideas: cool locations, encounters, npcs, plots, background etc. and when I actually get to the part where I decide how many monsters should be in this fight and what their loot is and such that part just takes a few min and occupies a line or two in my notes.

I'm sure it will get a little more complicated at higher levels, especially with spell casting monsters, but so far I am quite impressed with how little of my time is about the mechanics and how much of it is about making an interesting story. I could easily see running a session off a page of notes and a second page of monster/trap stats for quick reference. My main inspiration for writing stuff out more is that I want to have story arcs and recurring NPCs and locations and want those things to not contradict each other because I forgot something important.

 

[KirayaTiDrekan]

I DM this way as well, though I generally don't have any notes, keeping it all in my head. The thing is, as someone else mentioned, I am nearly constantly paging through the books, thinking about the upcoming session, and "prepping" for hours before a session - basically, whenever I'm not doing real life stuff or intentionally doing something else with my leisure time (watching a movie, playing a video game, etc), I'm thinking about gaming.

It usually goes pretty well, but there have been times when it just doesn't click - for whatever reason, my improv skills are off that day and the session turns in to a dud.

The most time I have spent prepping was when I wanted to use minis and maps (I usually don't) and spent a few hours setting up a battle map with dungeon tiles or whatever. The main reason I don't generally go that route is that, at least with my players, the game goes from "I charge in with my battle axe, yelling a dwarven warcry" to "Somebody move my dude up to that funky looking orc so I can hit it."

 

[Rocksome]

That's how I've been doing it since 3.0.
I get down what I want to achieve (where I want the PCs to be for the finale), then list a series of encounters, and outcomes I am hoping to achieve.

I might write a small descriptive sentence and a note about a particular "thing that happens", and possibly some page numbers of MM entries.

I find my players respond better to this, as it makes them think they've "gone off book" and they feel less like they're being railroaded.

I find that long block text entries shatter the illusion that your PC has free will.