Take the Ziplock and fill it with water. If you use a double seal bag then cut off the first seal.
Step 2: Close it all up
seal the bag but leave a very small gap between the seal. if you leave i open too much it will leak and give away the prank. if you seal it too much it won't burst open. it is okay if it leaks a little. if you don't want to risk it you can try using one strip of scotch tape to temporarily seal it.
Step 3: Placing
Now place it dirrectly under the pillowcase. wait until morning........ your victim
should have a nice and wet bed! Have fun :)