How to make a 2D origami ball (balloon)

(page 10)

17th picture of 2D origami ball (water bomb)

17.

Turn the origami ball model over.

Prepare to apply two "inside reverse-folds" .

18th picture of 2D origami ball (water bomb)

18.

Apply the two "inside reverse-folds" of the step #17.

Prepare to apply a "valley-fold" .