My players are pretty much at the end of Death Frost Doom. They're probably going to want to head back into the dungeon, so the horror of an unwritten 4th level is staring me in the face.
Going back a few years, check out this post on keying level 1 - I still follow that process by counting the total # of rooms and using that to calculate how many of each different encounter type I should have.
I have a big Word document full of ideas that I haven't put in the dungeon yet. I cull some for being too stupid (choco-medusas, you are DENIED), delay even more to deeper levels, and then assign the tiny remainder to rooms in the 4th level. Finally, I'll mark off the "monsters", "monsters w/ treasure", "specials", etc. that I've used, so I know roughly how many more I have left.
Then - I randomly write little notations in all the unassigned rooms ("M" for monster, "MJ" for monster with treasure, "T" for trap, etc), until I've assigned everything. None of it makes sense now, but it's a starting point.
As I go through writing up each room, occasionally the placement will strike me as ludicrous, and I'll move things around until it works better. But when you've got a few hundred rooms staring you in the face, you have to start somewhere - and for me, it's with this process.
Finally, I figure out how much treasure to put in the entire level by taking the amount of XP it takes a fighter to go from 4th to 5th level, multiplying it by 5 characters in a party, and then multiplying again by 0.8 as roughly 80% of XP comes from treasure, and then doubling because I don't expect players to go through more than half before heading down to the next level. That total number will get divvied up among the dungeon - I'll keep a countdown as I add treasures to the encounters.
All this is a long-winded way of saying I'm actually working on the 4th level now, for those of you wondering when I'm going to publish it...