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.