Here's how I do it

steal a hexmap somewhere online, google "hexmap" until I find one I like
get Zak to send me giant hexlists years ago so I don't have to do all the work myself and copy paste them in over half a decade or so
spend a long time in 2013 using Logan Knight's random generator factory at http://www.lastgaspgrimoire.com/generators/the-seventh-order-of-the-random-generator/ to make a hex generators 
forget that I did that and just write a bunch whole-cloth around where my players are
then use cedric howe's hexkit program to completely remake my hexmap, and then accidentally save over it, and facepalm in the middle of a game when I try to open it up and realize what I did, and just go back to using the random janky online one I started using in the first place
when they say "let's go to X place where there's a dungeon" I go "time to write a dungeon"
go to http://donjon.bin.sh/fantasy/dungeon/ and mess around til it makes a dungeon map I like
google castle maps until I learn that dyson logos himself made a useful numbered keep on the borderlands map
find an online copy of keep on the borderlands and re-write everything to make it weird
literally everything actually
give every NPC a blurb or two to say when PCs interact with them.
for instance: "smithy and armorer DARKBAN OPEN, a skinny guy with upset stomach . . . 'THIS TORTURE EQUIPMENT IS DEPRESSING THAT I NEVER KNOW WHAT ITS BEEN USED ON.' " or "Provisioner. BLAD OKLEBY. . . . 'I WISH I HAD A WAY TO SEE THE SUN IN HERE' " 
this is so if I'm too drunk or at a loss there's something I can blurt out
I don't know where I get the names
let's see what else
I try to put treasure and secrets everywhere, sometimes nested. so there's secrets hidden inside secrets. and I make it so some NPCs want something specific and concrete and some want something abstract and some want something random ("I LOST MY LUCKY DICE IN THE SKULL CRATER EAST OF HERE WHILE I WAS ON MY EXERCISES.”) and some want something that can't be fulfilled and some want something that can be easily fulfilled. so there's plenty of variation. and I try to make it so nothing found is purely helpful, there's almost always some kind of catch. and I try to make it so no one is purely good or sane. and I try to make it so there are multiple ways in to everywhere, but there is no no-cost path. every path has some kind of cost--it's about choosing which one, and finding the most fortuitous way at the moment, which takes luck.
like for instance this prison is layered. there's a part of the prison that's more or less public, it has an inn and a smithy and a tattoo shop and a wizard and a cleric and stuff like that. and then there's a more private part where the warden and guards live, you have to get special clearance to get inside that. and then inside that part is the actual dungeon, with its own layers. there are multiple paths into all of these layers, but the most obvious path is in this case heavily guarded. my players opted to become prison guards (their idea) and went through an application and interview process. but there are other ways in.
anyway then I have a like 80 room dungeon and I just go into a fugue state and write it
here are some techniques I use:
go to a coffee shop and get really caffeinated and handwrite rooms
when that fails I write directly on the map
or I open up a jpeg of the map and write stuff on it
for this one I had been excited about Zelda and decided to put zelda type stuff like a BIG CHEST and BOSSES and a BIG KEY and stuff like that, so that got written directly on the map and the ideas are folded in
and I know who the leaders of the multiple factions are, so I think about the implications of their factions and write that down and use it as a guide
then I make a random encounter list that has some really deadly possibilities and a couple bosses and and a few nothing entries and also some really weird stuff they haven't encountered yet
and then.... I just write it down in the text file
and do the "one more thing" technique when the going gets tough
and also do the "anything can be inspiration" technique when the going gets tough
and the "actual research provides insane ideas you would never think of" technique also when the going gets tough 
(i would never have come up with the idea that the hand of glory candle can only be put out by milk)
and I hand-draw diagrams of complicated rooms, of course, but generally try to keep it to "one thing per room" since this is an online game and it's pretty hard to do complex rooms online
but google piranesi pictures because this is a gigantic labyrinthine fantasy prison and it would be just be a big fat missed opportunity if I didn't have insanely complicated and beautiful etchings to show my friends with stuff all over it they can point to it and go "what's that" and I can say "oh that's an ice sculpture with a brain at the middle of it"
and put secret treasure and magic items and gold and spells and secret doors and magic doors and weird NPCs and friendly NPCs and friendly places and totally hostile places and traps and mysterious stuff with hidden explanations, so that it feels really textured
the idea is to make it varied and textured. you want it so you never really know what's going to be in the next room, but it also sort of makes sense, and so that stuff that is similar is similar in a way that echoes pleasurably. like how the violence in twin peaks is never the same each time, but always reminds you of itself... or not
and it needs to feel unfathomably old, and like no one else really knows what's in there, or really cares, except maybe for the witches. witches always care about stuff like that.
and then i just steal ideas whole cloth from zak and jeff rients and patrick and scrap because, i mean, they're great ideas
and then I just do that over and over again between sessions until it's just insanely complex
honestly I think I'm too easy on my players. I could stand to put more treasure in there though, I'm pretty stingy still after all these years.
also the ways I make dungeons and adventures changes each time. I don't know what I'm gonna do for the next one. this is just how I did this one.