Urban campaigns don’t have the easy execution of traditional dungeon crawls but they’re not an ineffable mystery, either. If I were in your shoes, here’s what I would do.
Pick a city. For optimum simplicity I’d pick an old, rambling, not-entirely-coherent city from fantasy fiction that you’re already familiar with. Lankhmar is a perfect example. Then change the name and just keep the feel and flavor; you don’t want to burden yourself with the difficulty of matching the details to the fiction. You just want something that can give you an idea of what the city feels like and provide you with tropes you can use when you need fast flavor.
Pick a neighborhood in the city. A shabby slum with leavening of crime bosses, corrupt officials, dubious priests, and suspicious foreigners is a good choice. Explain to the players that for the first few sessions you’re just going to be concentrating on activities in that neighborhood so you can keep your NPC roster and local activities manageable. The PCs aren’t trapped in there, but they shouldn’t expect local events to spill over into the wider city until you’ve had more time to get comfortable with the campaign.
Do a quick cut at a couple of opposing power structures: “Criminal Gangs” and “Local Officials”, for example. You can create more of these power structures as they become relevant in play. You put one person at the top of the structure- the biggest local gang boss or the local watch captain, for example. Beneath them you put two lesser figures, like smaller gang bosses or patrol lieutenants. And beneath them, you put four street-level NPCs that the PCs might ordinarily interact with, like typical gang members or local watchmen. At this stage you just give each of them a name, a distinguishing characteristic, and a rivalry/feud/friendship/debt for somebody else, either in the same structure or a different one. These relationships are important because they’re cheap and easy plot fuel when you need to explain why one guy is trying to get another guy killed, or why NPC X is willing to get help for NPC Y.
Now go to the one-page dungeon repository and start pulling maps that you could plausibly reskin into: Fortified Estates, Slum Warrens, Infested Sewers, Long-Buried City Ruins, Haunted Shrines, and other suitable urban areas. When you reskin these maps, just turn the humanoids into local citizens or denizens and drop any of the animals or magical beasts that wouldn’t be appropriate for the area- or turn them into human guards or the like if that’s appropriate. What you need from these one-page dungeons are just references- you can turn subterranean passageways into cramped alleyways and cavern rooms into tenement apartments.
Once you have these ingredients- power structures with NPCs and maps that you can reskin into interesting places- you just pick a conflict between NPCs and make that the evening’s adventure. Somebody wants something stolen, somebody killed, somebody protected, somebody rescued, something sabotaged…. Put it or them into a map, let the PCs negotiate with the locals on their way through, and then let their actions and success/failure shape the local conflicts and NPC attitudes toward them.
As a side note, don’t worry about mapping the actual city or neighborhood. Grab something online if it looks nice, but in actual play most PCs are only ever going to care about their immediate surroundings, which you can fake with a one-page dungeon map that you treat as roads/courtyards/buildings.
– Kevin Crawford
The question was asked multiple times and he got somewhat more comments here and here. I’m currently running The Wererats of Relfren by Grant Boucher and William Kurt Wenz, from Dungeon #14, p. 48–62. I’ve removed the silly and replaced the important people and locations with things from the starting village of my own campaign. It has been working very well until now!