- Collect all dungeon maps available (Browse through your stack of unused adventures.)
- Mark doors where they connect.
- Let dungeon-life begin.
- Use any simple random generator to create a background story.
- Let players roll up characters quickly.
- Toss them in.
But then again you can create a megadungeon that way. Think of it this way: If you'd take your average campaign, skip most of the wilderness parts, and cut the go to town shopping-sequences you'd end up with a collection of dungeons that are somehow connected.
It's old school-ish. It's simple. It can be fun.