The Waterbomb can be folded with any paper, you just need to cut it into a square shape.
Sometimes this model is also called the origami cube, or origami.blow-up cube because you need to inflate it at the final step. Sometimes also the term waterbaloon is used. But lets not get ahead of ourselves.
You can use the video above or the diagrams in the picture below. Additional
by the way: the diagrams or under a Creative Commons Attribution-Sharealike License. That means you can use them free of charge in schools and even make copies!
bring the two marked points two the middle of the lower edge. Then push the shape flat
take the upper of the two flaps on the right hand side and fold the tip towards the tip at the center
the new 90° Corner of the top flap should now be folded to the center of the model
take the tip of the folded flap and fold it down
push the corner inside the pocket
repeat steps 5 to 9 on the other side
fold the other two remaining flaps like in steps 5 to 10
fold the tip and the bottom end of the model firmly towards the center. then unfold
puff air inside the chape through the opening. one strong puff works better then slowly inflating the baloon.
enjoy your origami baloon!