[Edit - Refined and PDF'd version here]
Game playtested with my friends a single time - so take with a grain of salt. It's sloppy fun though, and part of the point of it is to end up with play artifacts that can be used elsewhere.
Requirement: an understanding of how some D&D concepts work (DCs, combat mostly). It's nice to have a reference for average AC, HD, damage, and DCs as well for folks less familiar.
Start the game with a stack of index cards. Each person makes (should take < 15 minutes in the making phase):
- A class + starting item (at a minimum, HD and level-up requirement [kill three snakemen per level, etc], but trapfinding bonuses, attack bonuses, special abilities are nice)
- A piece of loot
- A spell (either go Vancian with your spellcasting or have a spellcasting DC on each spell, detail what happens on success or failure)
- A monster (HD, attack bonus, AC, damage, you know the drill) [probably start with 1-3 HD monsters, go up 1-2 HD with each playthrough)
- A trap (needs a DC to find the trap, plus what happens when it goes off)
- A room (basic quick sketch, links up to some or all of the 4 sides of the index card, can have Things and/or Monsters in them, which will randomly be drawn from the created stack of cards, plus whatever special things/rules are there)
You can't start with your own class, randomly draw until you get a class you haven't played before and isn't your own. Write up a one-line character on an index card, no need for ability scores. Shuffle the stack of rooms, stack of monsters, stack of spells + loot + traps. Draw a room, for each monster symbol draw a monster and deal with it, for the other things try to search for traps before flipping over the card to see what it was. When the monsters run out, reshuffle that deck. If there are a bunch of people playing probably draw two or more per monster symbol. Flip over new rooms from the deck when you explore through the exits. When you're done with the dungeon go back to the making things phase and add them to the pile.
I had spellcasting working as a pool - you started with Xd6 in your pool and expended portions on casting spells, you could add in red dice to the casting that did damage to you equal to what they rolled, which was maybe too brutal but hey.
I probably didn't explain everything, but y'all probably get the gist of it and will want to change parts anyhow.
The game has like 0% chance of actually being balanced the first playthrough but that's part of the fun. Our testing found us comically underpowered at first and dying after finding a useless piece of loot, crawling through a shitter, and being murdered by the first monster we encountered. Eventually some folks drew OP classes or weapons and they actually managed to survive to level up, and it got easier. Probably best if you retire adventurers at level 3 or 5 maybe, since the dungeon doesn't scale as much.
It's fun, and you wind up with a weird assortment of monsters, classes, spells, etc. We had lizardmen, dryads, buzzsaw rippers, terrible wizard apprentices, etc as PC classes, and weirder things as enemies. Like most games, probably best played with a drink in hand.