I like to design adventures for particular players and PCs, rather than generically. What kind of adventures do they like? Hack and slash, puzzle-solving, lots of NPC interactions and character building, or a combination? A crew of action-craving players isn't likely to enjoy a murder mystery at a dinner party, and their PCs won't be very effective. I have one player who likes cats, so there's always a cat involved somehow in her party's adventures.
Once I've decided on a general feel for the adventure, I begin mulling over each element of Orson Scott Card's MICE (Milieu, Idea, Character, Event) quotient for writing fantasy or science fiction. Usually one element comes first, from the same place any idea comes from (your guess is as good as mine) and the others follow from there. The first element I think of often becomes the most important, probably because it was my source of inspiration for the others.
Milieu: What is the setting? A dungeon? A city? Wilderness? Which of these settings will provide the most opportunities for the PCs to use their skills and class abilities? Which will they find most intriguing?
Idea: This is the "what if" element. What if an evil being put all his power into an unassuming-looking ring, subsequently lost it, only to be found by one of the PCs (a halfling, perhaps?) who had no idea of its power?
Character: Who is this evil being, anyway? What does he want? How will his actions first touch the PC's lives? Who are his followers? Does he have allies? Enemies? I usually design the biggest, baddest villains first, and then fill in their henchmen.
Event: What do the PCs have to do to solve the problem posed by the "what if" idea? Hurl the ring into a volcano? How do I make their quest clear to them, so that the adventure won't go off-track? Will bad things happen if they delay? Will an NPC wizard warn them of the danger of the ring? Will the evil being send his minions after it?
Once I have these four elements in place, I write a paragraph to myself explaining the setup for the adventure, just to make sure it all makes sense to me. Think of it as a synopsis that no one besides you will ever read.
Then, the time-consuming and tedious (for me, anyway) part begins: statting the NPCs, drawing maps, coming up with puzzles to challenge the players. I probably enjoy improvising more than most DMs do, so I leave some details sketchy in order to fill them in as the adventure progresses -- my players often unwittingly (or wittingly, I suspect, as they have come to know me) improve upon my original ideas as they speculate aloud about what's going on. My general philosophy is not to design one bit more than I have to. Who cares what the innkeeper's name is? If it comes up in play, I'll just make up a name on the spot: Barliman something-or-other.
The last step is figuring out where the PCs are when the adventure begins, and how to work them into it in the most plausible, economical way possible. "You're sitting in a tavern when..." Again, sometimes the players can be helpful here. I often start an adventure by asking them, "It's a typical day, at 10 AM. Where would your PC be and what would they be doing?"
Hope this helps. I'd love to hear how other DMs work differently.
See my sig for an example.
