http://www.pencilkids.com/monkeygohappy4game.html 

You have to dig a hole, about four blocks deep, exactly three blocks wide and four or five blocks long.

Then you break two blocks two deep at one of the short sides of the hole.

Place a red block there.

Put a layer of lava at the bottom of the hole.

Then place a sign on this red block, then another sign on the first sign, and another and another, as many as you can.

Place signs on the side of the existing signs. Now cover up the hole above this framework of signs.

You can try your trap now, just walk there and hit the red block.