How to make an origami butterfly based on open sink-fold

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1st picture of origami butterfly

01.

Let us just start with "balloon base" .

If you don't know what balloon base is, study the above link to preapare this base.

If you are ready with balloon base, then prepare to rotate it by 180 degrees.

2nd picture of origami butterfly

02.

Rotate the base by 180 degrees.

Prepare to apply a horizontal valley-fold at 1/3 height from the bottom.