Wait until your intended victim is asleep or out of the house. Nighttime is the most ideal time to do this, because your target probably won't notice the saran wrap with the lights off. Even if he does turn the lights on, he'll most likely be half asleep and won't notice it.
Tear off a couple large sheets of saran wrap from the roll.
Lift the lid and seat of the toilet.
Cover the toilet bowl with several layers of saran wrap. It's better to wrap the toilet from side to side, rather than front to back, so the victim doesn't notice anything amiss.
Wait for the victim to use the bathroom and enjoy his intense reaction.