How to create a Megadungeon in 2x3 steps?

Step 1

1. Collect all dungeon maps available (Browse through your stack of unused adventures.)
2. Mark doors where they connect.
3. Let dungeon-life begin.

Step 2

1. Use any simple random generator to create a background story. 
2. Let players roll up characters quickly. 
3. Toss them in.

Of course, you may say: "That's not a great idea." And some of you will. And I agree.
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.