I did something like this last night. I used the pant front and back pieces, shortened them, then also used the skirt piece on the front only. I extended the pant front so that it had square rather than notched in corners and omitted the side-front panel. I modified the facing for the pant front. After hemming the skirt front and doing the buttonholes, I basted it to the pant front and treated it as one piece. For the elastic I just followed the directions as written because there was only one pant layer in the back and not bother a pant and a skirt layer. I didn’t really think through the shape of the pant leg when I shortened it so I had to fudge it a bit when I was hemming it, but I think it turned out reasonably well. I still need to add the buttons and finish the elastic, which I will do when my daughter is up in the morning to try it on.