This is the front side of paper.

It is somewhat shinier compare to the back side.

The crease pattern (CP) for balloon base is shown on the left.

The order of folding is

(1) two valley-folds followed by

(2) two mountain-folds.

The following steps #1 through #8 explain these folding in detail.

The CP for balloon base is exactly opposite of the one for square base. If you switch between valley-fold and mountain-fold, then you can switch the base.

1.

Let's start with a paper's front side facing up.

Prepare for a valley-fold along the mid horizontal line.

2.

Apply the valley-fold of the step #1.

3.

Prepare for a valley-fold along the mid vertical line.

4.

Valley-fold of the step #4.

5.

Prepare to mountain-fold along a diagonal line.

6.

Apply the mountain-fold of the step #5.

7.

Prepare to mountain-fold along another diagonal line.

8.

Mountain-fold along the diagonal line.

Prepare to touch four corners with your fingers.

9.

Touch each corner with your finger.

Prepare to move your fingers along the arrow directions.

10.

Flatten the model.

11.

Put the model down.

Balloon base is now completed.