First, make a five by five size house
Build it up to three blocks.
Then put signs all over the top of the house.
Then put a diamond block in the middle of the house.
Then add signs on top of the signs like so...
Then cover the roof with sand. (Except the middle were the diamond is.)
Then cover the middle hole.
Then put another piece of sand on top of the middle.
Then put up a sign that says:
When they walk in and break it,
The sand falls on the player!