First take a piece of fabric wide enough to fit loosely across the child's shoulders and allow them to take steps. This is one piece of shirting fabric, folded lengthwise so the selvedges are together.
Fold the fabric in half widthwise. The centre fold will be the centre of the front and the back. Cut a small piece out of the top folded corner. This will be the neck hole.
This is where the serger comes in handy. Serge the side seams, leaving a gap at the top for the arms. Serge around the neck hole, the bottom and the arm holes. Trim and singe the threads.
Wear over a jumper and a pair of tights, with a tinsel belt and tinsel halo.
I didn't make wings.