Start by placing your button down shirt on your body form. Then trace a line around the top of the shirt around the shoulders. This is the line that you will cut for the new "off-the-shoulders" neckline of your puffy shirt.
Take the shirt off of the body form, and cut off the top of your shirt by following the line.
Next cut off the bottom part of the sleeves, at about the three-quarter length.
Then fold the top edge of the shirt in about 2 inches, and pin, to create a casing.
Sew along the pin line using your sewing machine.
Next cut two slits in the middle of the casing on both shoulders, for a total of four holes.