A french seam is appropriate on sheer fabrics, such as chiffon and organza, and is also sometimes done on rayon batiks. French seams are done on straight seams -- for curved seams (such as armholes), a bias bound seam, or hong kong finish is appropriate. French seams are also seen on many heirloom garments, with tiny 1/8" seams.
sew seam, wrong sides together, with a 3/8" seam allowance (I am assuming 5/8" seams). Trim seam allowance to 3/16".
turn and press seam, enclosing raw edges. stitch with a 1/4" seam allowance
I like narrow french seams -- this one is 3/16" wide. This is the wrong side.
Finished french seam, right side. Even with the contrasting thread, it looks pretty good!