Strange as it may seem for the grognards among us, Keep on the Borderlands can be somewhat overwhelming for the first-time DM. The module packs an awful lot into a small number of pages, and just running it alone still means dozens of NPCs to detail, not to mention the intrigues taking place in the Caves of Chaos. Of course, most of that is entirely unnecessary, and the players will never experience a fraction of the material you have prepared for them.
So don’t prepare it.
Don’t think of the module as a burden, something that you have to hack to pieces before play, something you have to add infinite detail to. Instead, treat it just as you would if you were creating your own setting, and work out only what you need to work out. After all, the PCs will not be going everywhere at once; what you need to focus on are those NPCs and locations that they are highly likely to visit, and the DM has at least an element of control in narrowing this down.
You need to know who rules the Keep, but at this point, just a name and a vague description will do. Unless you as a DM mandate it, a fresh group of wandering rogues are not going to get an audience with the ruler, not until they have proven themselves. (I usually use Morgan Ironwolf, as a tie-in to the Basic Set; the backstory being that she has retired from her adventuring days to take up her role as Baroness of the Borderlands. I often drop some of the other characters from the Sample of Play into the Keep as well, in various roles.)
None of these need to be fleshed out in anything like exhaustive detail at this stage. Names, a few one-word descriptive ‘tags’, and you’re done. Don’t put more effort in to characters you don’t know that you are going to use. Most of those are likely, but not certain, not by any means. You may have a few other ideas as well, NPCs that you hope to use in future adventures, perhaps tie-ins from other modules, feel free to list those as well. Again, tavern patrons. At this stage, all you have is a single sheet of paper, and that’ll be enough.
The trick being that you are going to flesh these out during play. For the first session, this is all you should need. (Actually, I usually spend the first session getting to the Keep, letting the PCs get to know each other before throwing them into the larger world. Perhaps one NPC from the Keep to serve as an early patron.) You’ll quickly work out what you need, and which NPCs can remain just a name on a piece of paper and which you will be fleshing out. Remember that you do not want to drown your PCs in detail. Throw twenty NPCs at them and I guarantee that they will not remember any of them. Keep it simple at this stage. (That, or put together a directory for them to follow. A list of names and shops at least. Though if you limit the introductions at the start, that ought to help.)
Don’t be afraid to change the Keep as well. Perhaps the Trader went out of business and there is now an empty storefront, which can be taken over by another NPC if you realize the PCs are going to make frequent use of a service. I’ve done this on a few occasions. And when it comes to the Guild House, again, that can be left empty. Perhaps the Guild has failed. Or the PCs are the Guild; I’ve done that a few times. Again, it’s a good idea to see what the PCs are likely to require. A party with multiple Magic-Users will justify some sort of Mages’ Guild.
Now we get to the Caves of Chaos, presumably early on. Don’t even think about covering all of them on the first crawl. This has to be focused or frankly you are going to kill your PCs. Pick one of the easier dungeons and give the PCs an objective – rescue a prisoner, retrieve an item. Make sure they have a goal, and give them a guide. Perhaps an NPC who will get them to the cave, even accompany them if needed, or alternatively a map. You could even use a prisoner, a captured goblin. Introduce them to the caverns as you’ve introduced them to the Keep – slowly.
The rest will come. Twelve sessions into the campaign, you’ll have a folder full of notes that you have prepared organically, and your players won’t need a study guide to find their way through the Caves of Chaos.

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