At least for now. This'll probably change, as I get better at it.
First, spend a couple days working on the map. Usually in class, because my teachers don't care if I "doodle."
Once I've got the map, I break out some lined paper and start keying, ten rooms to a page. Sometimes I just put stuff in the rooms that I think would be cool. Other times, I use Sham's Restock Tables to populate them, usually with a second die rolled for a 1 in 6 chance of a trap.
If I roll a monster, I roll on the Swords & Wizardry tables to figure its CL, and either invent one or pick one out of the S&W bestiary. Then I add it to the Wandering Monster table for the level, along with its stats, and roll number appearing for the room in question.
For traps, I either come up with something on my own (not great about that yet, but I'm learning) or a roll on Sham's Trap Tables. In either event, I spend some time thinking about a triggering method and warning signs. I'm trying to break out of my "traps are abstract, random damage" habits that kept me from using them in my 3.5 days.
Rooms with treasure often just get something I think would be cool. Otherwise, I roll on the S&W tables, or stick cash somewhere. I'm not real great about hidden treasures yet, mostly I stick things under loose bits of flooring. (Or on the ceiling.)
Empty rooms are rarely completely empty, and even rooms with something mechanically noteworthy in them get some bit of flair or dressing. So far I've got a bunch of mosaics on the floor, a room with poetry all over the walls, and spawning pools. So far I've only got one "trick," a dungeon feature that isn't strictly a trap, but still might be dangerous and takes some figuring out, but I'm going to add more as the mood strikes.
Oh, and all of the rooms have names, to help me remember them. Most of these are strictly utilitarian ("Music Hall" "Control Panels" "Beehives") but a few are whimsical or referential ("Fiery Doom" "It Has a Sense of Humor") because I enjoy amusing myself with that sort of thing while I prep. (I once named a monster "The Thing What Lurks in the Swamp" and nearly collapsed laughing as one player explained to another player their plan regarding said creature. Poor form, I know.)