How To Make A Dungeon
First you need a map. Go use watabou's generator. Turn off the notes and objects and stuff, you just need the rooms. Make it smaller than you think you should—fewer rooms, I mean—players always find dead trails to follow. If this is your first dungeon, shoot for maybe ten or twelve rooms. Print it out or something.
Ask yourself, "What is this place like, in general?" Think of one or two broad descriptors: hot and humid, cold and damp, dusty, crystal walls, moss-covered, whatever. Write it at the top of the piece of paper: "This dungeon has a low ceiling and dirt floors." You could say what the dungeon used to be, instead: "In times past, this dungeon was a secret underground monastery for a lost sect of templars." Don't write more than one simple sentence. Use that to inform the rest of what you write.
Make a random encounters table. This will set the tone and function of the dungeon. It makes it live. You only need three entries: a common encounter, an uncommon one, and a rare one. Big encounters tables with lots of entries are fun and cool, but small tables make a place feel more cohesive—we're going to use that to our advantage. The table could have monster encounters, an NPC with an unknown attitude toward the party (that's what reaction tables are for), or even traps. Arrange them in a table.
1-3. Goblins.
4-5. Trap (1-4 pit trap, 5-6 poison wall darts).
6. A lost wizard (Gorpix, level 3) with entourage (6 level-1 fighters, and 2 porters).
Now find the entrance to the dungeon. The room it empties into is Room 1. Write "1" in the middle of it on the map. Below your random encounter table, write down what's in Room 1. Make it simple and short, shoot for mysterious and moody if you can. Take it slow, no need to front-load all your best ideas. Make them work for it.
- Piles of crates and large barrels, moldering against the back wall. They're full of spoiled foodstuffs and vinegar that was once, probably, wine.
Okay, great. Now you're just going to do that a few more times. Some rooms should have more interactable things, but not too many. Overall, you only want to key maybe 60-80% of the rooms, and only maybe half of those should be someone or something to talk to or fight or investigate. You can do more than that, but in my experience you don't have to. Remember that your random encounters table is going to pull a lot of that weight—let it.
It's probably good to put an encounter on the map for each type of creature on your encounter table (for us, that means putting some goblins in one of the rooms). Use that as an opportunity to establish them with a motivation of some sort for being here. Maybe it's a goblin nesting room, with nine goblins and beds made out of sacks of toenails. Maybe it's six goblins holding a girl from the local village hostage and forcing her to sing that bawdy song they like. Don't overdo it, though. Just the one goblin room will do—again, the random encounters table will take care of the rest.
Once you've got all the rooms done, you can consider adding one or two keyed locations in the hallways. A trap, or a gelatinous cube, or a big colorful tapestry, or a dead goblin or something. But only if you have a good idea, don't sweat it if nothing comes to you.
Hopefully you're using a bestiary that takes care of the boring stuff like Number Appearing and Treasure, but feel free to add or change those things as the mood and inspiration strikes you. Maybe the leader of those 2d10 bandits in Room 9 has an eyepatch and carries an antique silver handheld mirror. Lean into one or two small details with no inherent meaning. Instead, let the dungeon accrue meaning as the players move through it. Make sure to repeat ideas in new contexts—it's easier than coming up with new ideas all the time, and it becomes a "theme." So don't be afraid to put an antique silver hairbrush in the pocket of the Goblin Mother in Room 14, as an oblique reference to the goblin leader's mirror. But for the love of God, don't try to make it make sense. I repeat: Do Not Make It Make Sense. The players will either do that on their own, or they won't care enough for it to matter.
Avoid the impulse to be clever. Be lazy, be foolish, be anything but clever. Only look at what you're making from the corner of your eye—it makes it easier for the really good ideas to sneak up on you. After a while, you'll look back at what you've written and see connections and themes and all kinds of things you never intended. That's good.
Having said that, you should always include one encounter that you're really proud of, something you can't wait to show your players. Most of the time, your goal is to be surprised by how your players explore your dungeon, but sometimes (less than you think, but occasionally), you should surprise them, too. Really keep them on their toes. That's what keeps the simple encounters like 2d6 goblins interesting: they know what you're capable of, and it could happen at any moment. Nothing is ever fully safe or predictable, even when it is. Write this encounter last, that's when you have the most established symbols and ideas and themes to draw on.
Go quickly. Don't think too much. If you stall out, go smoke a cigarette and don't bring your phone. Or eat a baby carrot, I don't know. Once you've got it all done, put it away immediately and stop thinking about it. Just drop it somewhere on your overworld map and forget about it.
Oh, you don't have an overworld map yet?