Mark it out with the usual Polygonal Lasso Tool, make a copy of it and place it above the girl's layer. Make a lot of copies of this layer (ctrl+j), changing the size, the angle (Free Transform) and the butterfly's level of distort (Edit ? Transform - Distort). Place the copies on the girl's dress and on her skin.



Combine all the layers containing the butterflies (ctrl+e) and make the corrections on the color (Selective Color).



Make a copy of the butterflies' layer to make the copy entirely black, using Image ? Adjustments ? Channel Mixer.



Place this layer above the girl's layer and group them out (the shadow shouldn't get outside its contours): Layer ? Create Clipping Mask (ctrl+g). Blur out the layer (Gaussian Blur).



Make the Opacity smaller till 80%.