I just cut the jersey off under the arms to make a rectangle open at two ends.
Then I just zigzagged two strips of elastic to the top of the rectangle. All the other seams are already finished, and the zigzag on the elastic should stop the top of the skirt from fraying too badly.
I had to use underwear elastic because that's all I had, but J didn't seem too worried about it.
It certainly did the job for her Red day (in honour of Red Nose Day), and she wore it all afternoon.