I have used this two-step seam method for most of the seams in the Elegant Lady's Closet drawstring dress.
Sew together the seams at 15mm as indicated in the pattern instructions. Open out the seam and trim back one of the seams, in this case I've trimmed back the fabric that was attached to the 'lining'.
Fold the outer edge in over the narrow edge and crease.
Fold again and pin.
At this point you can do a plain hem stitch.
I did a simple running stitch that suited this garment.