I started with this button down ladies shirt, there was no size marked in it but I think it was a med or lrg.
I cut straight across right under the arms.
Get rid of the top part for now, you can use it later for something else. Flip the bottom part inside out and press down 1/2 inch, stitch.
Flip over 1 inch, press and stitch, leave an opening 2 inches long. You are forming a casing for your elastic.
Measure around your child's waist and cut a piece of elastic to fit. Attach a safety pin to one end and thread it through the casing. Note- use a wider piece of elastic than I did, it will stay up better on your child's waist.
Stitch the ends of elastic together and then stitch your opening closed.
Put it on a cute kid and let them do their best Wonder Women pose.