I had a small amount of black scrap fabric. I spent a couple of £s on some of the seam webbing. Here is what I came up with (modeled by my 6 year old daughter Isabella while wearing a witch costume dress today).
This is the shape you want to cut out, but in this picture, the top is already folded over, so remember to keep two longer pieces (with an opening for the middle) from almost mid-way to the ends of the wings. Those are the bits you will be folding over for your child's arms to slide through.
Here is a close up of the seam webbing I used (you place it where you want to "glue" the two bits of fabric together, and then iron it).
I did it all by sight, but I have tried to create a sample of the pattern shape I created when I made it. This is close, but actually needs to be deeper where the actual wings are. And I made the points at the bottom larger and less frequent than in my "pattern."
This post is created in collaboration with Creative Costumes.