Fold back the skirt facing at the notch, wrapping the zipper tape
seam allowances towards the back.
Sew the white triangles to the black triangles and press
the seam allowance towards the black sides.
Press
seam allowance towards the lining / facing unit and understitch (straight - stitch on the right side through the facing and seam allowances close to the seamline) to keep the facing from rolling out of place.
Not exact matches
Then, you need to pull the corners that aren't sewn together open, and pull the sewn edges
towards one another, essentially so the two edges with your
seam allowance are flush.
The
seam allowances get pressed
towards the back.