step 1: wait and stand around with an aloof, bored expression on your face. try to appear calm.
step 2: when everything is quiet and nobody is expecting it, jump up and scream at the top of your lungs.
this will ensure a successful surprise. make sure not to scream words like "bomb" or "fire" because this could cause unpleasant reactions from those around you and result in stampedes, panic, and general chaos. also, make sure that nobody around you is currently pregnant or suffering from heart problems.