Step 1: You Need...
-a balloon (the screaming kind work best)
-a couch or chair that your selected victim sits in
Step 2: Inflate the Balloon
After the balloon is inflated, use the clothespin to hold the air in.
Step 3: Set the Trap
Simply place the clothespin under the cushion so it will open and release the balloon when weight is put on it as your victim sits down.
Mine wasn't completely hidden, but it didn't matter because my unofficial and unaware prank tester didn't notice until the balloon went screaming across the room. It was quite amusing.