Inside reverse fold two sides in to the centerline. The finished waterbomb base has 4 triangular flaps. Unfold, so that the colored side is up. Valley fold , and unfold, vertically. Turn the model over, so that the white side is up.
Valley fold diagonally.
Unfold, so that the white side is up. Valley fold , and unfold, the other main diagonal. The white side should still be up. The finished waterbomb base. A traditional base.