When you are ready to use the waterbomb, spread the corners out, pulling on opposite sides to expose the hole in the top. Blow a quick gust of air into the model to inflate the square shape.
Here is the inflated waterbomb, which is a handy decoration, and fun for a pet toy.
This is the completed waterbomb model. It should be a soggy mess, and a big wet spot. Notice that my locks didn't completely give. This means I need to throw the model harder.
I hope you enjoy the waterbomb as much as I have. Just be careful, people that don't know how to fold a waterbomb will quickly get upset at you. But if you are battling with other origami lovers, make sure to get their paper stack wet before they get yours.