With a pencil, trace the pattern onto the muslin. Do not cut out the pieces yet. This pencil line will be your stitching line. Clip the neck seam as shown. Turn them right sides facing out.
Machine stitch the opening closed.
Stitch each to form a casing for the drawstring. With right sides facing, stitch the short sides together to form a tube, leaving the casing unstitched. Adjust the dress seam to be the center back. Adjust the arm seam on the sleeve so that it centered on the underside. Lightly stuff the sleeves.