I've had a pantheon, a world, and a butt ton of lore that I've made for personal reasons, so I jumped into DMing with my setting framed if not filled. However, that was years of work before ever actually playing, so I don't suggest that for the more green of DMs.

Mostly, you want to communicate with your players to find out what adventure they want you to write. Do they want to stem the orc hordes before they desend on the land? Do they want to track down a slippery criminal network that has stretched too far? Do they want to fight the gods themselves on planes the mortal eye has never witnessed? They'll tell you the destination if you ask, then you just gotta decide the route.

Aside from the grand scope of the campaign, I plan the adventure conceptually about 3 sessions ahead. I think about how the world, it's people, and it's villians would progress without the heroes interfereing. Where will they be or try to reach? Will they be getting more powerful? Who will they align with or make enemies of?

These thoughts are all nebulous and change more often then not. As far as the pen-to-paper planing, I start after the last session is done. I start by thinking of where the players are likely to go, who they'd talk to, and any side quest like tasks they may take off for. Between that and what the NCP involved want to do or can add, I make my maps for the places they'll definitely go and draft up a few spots where things may get hairy if they aren't careful.

Doing this, I can let the players wreck my plans while still having enough set aside to let them roll with it, all the while still keeping the story moving towards it's natural conclusion. Players don't mind getting railroaded so long as they have lots of tracks to change to, and I got a lots of separate tracks.

Typically, I like to get a "theme" in my mind, which could come from any number of sources. One of my players is in love with Fey stuff (stuff he's picked up from a lot of different media, probably most notably the Dresden Files series, Lost Girl, & Supernatural), but there isn't a lot of that material to pull from for this edition (so far). Then, when reading through the SCAG, I came upon descriptions of notable NPCs in the Feywild (I think it was in the Warlock section), and I started thinking about the idea of my players being unwitting pawns in a "cold war" between the seelie and unseelie fey factions, headed by their matriarchs Titania and the Queen of Air and Darkness (let's call her Mab). Both these quasi-deific figures approach the characters in their own way, inviting them into the Feywild and promising all sorts of fantastic treasures and power in exchange for "favors" that would help them get a leg up over their counterpart. The situation got my mind all in a tizzy thinking about the possible political intrigue, the non-combat encounters, the actual combat encounters, the forging and breaking of alliances, all while having to try to stay wary of the sing-song double-speech the Fey are infamous for. Not to mention, in the background, a powerful enemy is hatching a sinister plot the characters will eventually have to deal with.

Once I have the theme decided upon (a game of political intrigue with some of the most devious and cunning creatures in FR), I start thinking about specifics. Some of the questions I try to have answered, in no particular order, are:

- What would make the characters want to travel to the Feywild? More importantly, why would Titania or Mab be interested in bringing in outsiders? What have the characters accomplished to gain their attention?
- What kind of place is the Feywild? What kinds of quirks does it have? How do its inhabitants react to the PCs? What kinds of side-quests/additional narrative hooks can I create using this kind of setting? How does doing so help push my narrative? (The characters learn more about the different Fey courts, and how their subjects view their respective queen and the oppositions. The characters see how the dichotomy of the region is reflected among minor citizens, sees how they treat each other, helps settle disputes wherein they will have to favor one faction over the other, and see how the landscape [political and physical] changes over time because of such decisions.)
- What level should the characters be before they enter the Feywild? How do I get them from 1st to that level?
- What kind of promises would each fey queen make? (Tailor this to your players, and more specifically to their characters. Actively try to have each queen promise something at least one party member really wants.)
- Do I want to entertain the possibility that characters could end up individually siding with opposing factions (dear gods I can only hope so)?
- What other kinds of notable NPC's are there in this game? What are their personalities/motivations? What are they hoping to get out of the PC's, and what hook gets the PC's to cooperate with them (i.e., what do they have that the characters want)?
- What kinds of dungeons do I want to create, and what kind of thematic elements do they have?
- How much agency do I want my players to have in this story? How would their actions influence the primary/secondary/tertiary/etc. NPC's opinions and interactions with them?
- How much of a chance do I want the players to have to "lose," aside from the hopefully avoidable possibility of a TPK?
- Who is the BBEG? What kinds of abilities does he/she have? What CR is he/she? What kind of minions does he/she have? What is their plan, and what drives the characters to oppose it? What is their opinion of the PCs? What level do I want the PCs to be before challenging him/her in a final showdown? What consequences are there if they challenge him too early? Do I give them that opportunity?
- How many possible ways can this end? What's the "ideal" ending, what's the "worst possible" ending, and how many variations can I imagine in-between? (Again, leave some room for player agency.)

This is super free-form, I know. It's just how my mind works, and I open a Word doc file and just set to answering all these questions, and more as it starts to flesh out in my head. Also, obviously I'm kind of skating a bit, since (as I mentioned) I'm using the Forgotten Realms as my primary setting. If you're designing your own world, that's a grand, ambitious thing all on its own. Personally, I really (really) like the DMG's workshop for doing so, maybe one day I'll feel froggy and decide to give it a go.

So, back on topic.... After that, I try to do a rough "sketch" of a plot outline, detailing how the narrative flow of the campaign should go. Generating random encounter tables, deciding locations and types of treasure, detailing narrative encounters, etc. (I generally abhor using "scripts," aside from any specific wording I want to use to detail a person/place/thing; your players will always, always surprise you, so the more time you spend working on a dialogue script the more frustrated you're going to be when they veer left and go way off it.)

Other than that, I try to be extremely fluid with fine specifics. I try to provide a setting, interesting NPC's, exciting and memorable opportunities** for character interactions and combat, etc., but the players should feel free to play about within that framework. There is a middle-ground between railroading and aimless "Skyrim in D&D," and thankfully it's pretty large and forgiving. That being said, I'm not a huge fan of trying to crowd-source a campaign theme, because all your players want different things. Why try to cram it all into one hodge-podge campaign, when you could really focus your energies on making one great single-themed campaign at a time, and eventually cover everyone's wants anyway? :)

Historically I've done a lot of prep in advance, either adapting a published adventure or writing my own based on what the players did last time and where they're planning on going and so on. We tend to have a decent amount of time between sessions, so I have time to prep and I've done so sometimes down to the minutest detail. I tend to follow the 5-room dungeon format as a way to structure the story (even if we're not in a dungeon--it's a good way to make sure you have a good dramatic structure), and in the past I've been okay at improv but not always super comfortable. 

My whole approach to DMing has changed after running a few games of Dungeon World, however, and I'm becoming pretty comfortable with prepping way less and improvising a lot more. I'd be pretty comfortable bringing in a couple of possible locations and a handful of monsters I think might be cool and just go from there.

One thing that DW introduced that I think is incredibly helpful for campaigns is the idea of fronts. They are an excellent way to structure your overall plot (or plots, as the case often is and often should be) and keep things moving in the background as well as creating dramatic tension for the players without having to do a ton of extra work. Fronts are surprisingly effective, and honestly fairly easy to maintain. Basically you pick your Big Bad--maybe a faction, or a lich who wants to destroy life as we know it, or whatever, and write down the BBEG's motivation, the "grim portents" or steps that follow if the PCs don't intervene, and "the impending doom"--what happens if the threat isn't stopped. Fronts can work both for individual adventures as well as for campaigns.

Mike Shea (Sly Flourish) discusses how this can give you structure without railroading, and I find this particularly useful since pre-writing everything tends to lead to a feeling of railroading, even when you're pretty good at giving the players the illusion of choice. He also offers advice in a later article on how to simplify fronts and provides examples from the classic D&D adventure Legacy of the Crystal Shard. 

Utilizing fronts in my games has been a major revelation, and has totally revolutionized the way I DM.