To adjust the skirt the first thing that we need to do is sew the front and back darts. After pressing they should look something like this:
Using a basting stitch, stitch the two back pieces together starting at the notch and ending after the angle of the back slit like this:
Now sew the front and back pieces together.
Now for the tricky part - getting the fit right. First try the skirt on and pin together the back opening where the zipper will be (you could also just baste in the zipper if you prefer). Pin at 5/8" as that is how much the zipper will take it in.
Use pins to take in the skirt until it fits correctly.
Connect the pins with a line to mark the new seamline.
Sew on the new seamline.
Try on for fit.
Once the fit it right then measure 5/8" out from the new seamline and cut off excess.
Use the scrap you just cut off to adjust your pattern piece.
Now we are done with the muslin. Since I am adding length to my bodice pieces I am not going to sew the top and bottom together, but if you are not adding length then you can do that now to make sure the fit is right.