
Balloon base is one of two important simple base in origami. Equally important base is Square base. The relationship between Balloon base and Square base is like one between valleyfold and mountainfold.

You can start with any kind paper with a square size. Here the back side of the paper is shown. 
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 valleyfolds followed by (2) two mountainfolds.
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 valleyfold and mountainfold, then you can switch the base. 
1. Let's start with a paper's front side facing up.
Prepare for a valleyfold along the mid horizontal line. 
2. Apply the valleyfold of the step #1. 
3. Prepare for a valleyfold along the mid vertical line. 
4. Valleyfold of the step #4. 
5. Prepare to mountainfold along a diagonal line. 
6. Apply the mountainfold of the step #5. 
7. Prepare to mountainfold along another diagonal line. 
8. Mountainfold 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. 