Here is my method of finish a skirt opening in a french seam. Skirt openings are usually on the left-hand side of a skirt.
Attach the strips
Find the difference between your waist and your widest hip measure and add one inch. This is the strip length.
Cut two 1” wide by ‘strip length’ long of fashion fabric. Press short end up half an inch. With right sides together, sew one strip to each skirt panel with a 1/8” seam allowance.
Sew first part of skirt french seam
With skirt pieces facing wrong sides together, sew skirt panels together starting just under the edge of the strip with a 1/4” seam allowance. Press. Fold. Press again. Sew 1/4” (or 3/8” for a 5/8” seam allowance) away from folded edge starting at the bottom of the placket to the skirt edge. Press.
Press strips to inside of skirt. One strip will naturally press on its own seam. The other will naturally fold along with the skirt seam, so that the strip seam is in from the folded edge. The opening edges will overlap. Stitch down strip around three sides, going through the skirt seam.