- Make sure the place itself is interesting to explore.
- Don't overcrowd it.
- Allow for discovery, and re-use.
- Keep monsters mostly appropriate for the level.
- Place important monsters and treasures first, then if the place is too big, use random stocking to fill out the rest.*
Of course, Gygax assumes creation of a Megadungeon, similar to his Castle Greyhawk. By the time Mentzer was re-editing the Moldvay/Cook edit, more thought went into creation of smaller dungeons with specific purposes. But all of the above main points were still there. There's not a lot of actual advice for the crafting of the dungeon, other than the sample map and semi-explained key, and the example of play, though.
I do like the fact that in the sample dungeon, there's a 'special' which he notes is basically a Dick DM move to use, so don't use it.
Interesting differences from the later Classic D&D game include (old news to some of you, of course):
2 moves per Turn while exploring. I guess this was taken from Chainmail, with its split moves each turn to allow for reconsideration/reactions. 240' (120' encumbered) per 10 minute Turn is still really slow, but better than in the later editions.
Doubled chances to find secret doors when searching. Elves 1-4 in 6 (1-2 in 6 innate, rolled by the DM in secret), 1-2 in 6 for everyone else.
*I'm guessing that this picture of Gary's notes for Castle Greyhawk are so sparse because it's just the specials only. The rest he was able to simply roll randomly as he ran the game. Or maybe not even roll by the time this picture was taken.