This photo is on its side, but as you can see I added some sheer matching layers over the batting. It was actually the fabric scraps I used to test colorfastness. Then I simply tied the streamers onto right and left upper corners, to stream in the breeze.
At this point people often add straps of some sort, usually made with elastic. Living on opposite coasts from my granddaughter, I wasn't sure how big to make them. So I sewed three pinbacks to the center area. You can buy pinbacks at craft stores. This way the wings can be attached to any clothing. I also made a 4" padded square, made of fabric and batting, which can be used as a stabilizing base on the inside of thinner clothing, such as a t-shirt.
And there you have it! If I get a photo of my granddaughter wearing it (hint hint), I will post it here. Let me know if you try to make your own butterfly wings! There are all sorts of tutorials online, and this was my own take.