The waterbomb is an easy figure and it is to fold like the preliminary base.

The single steps of folding the waterbomb:

Step #1:
The first two foldings are quite easy. You only have to do two diagonally valley folds and turn the paper over afterwards. Please note, that the cornes are exactly on top of each other.

Step #2:
Do two more valley folds but horizontally and vertically.

Step #3:
Fold the upper and lower edges down (or do a diagonally mountain fold) and hold the paper in the air.

After you hold both outermost edges of the folding between your thumbs and forefingers push both edges together. The paper should interfold itself automatically.

Step #4:
Now turn the front edge to the right and the edge at the back to the left.