How to Sew a French Seam
When you want a seam with a clean- looking finish and concealed stitches, try a French seam.
Source: Martha Stewart
Align fabric pieces, wrong sides facing, and machine-stitch with about a 1/4-inch seam allowance. (Do not press seam open.)
Flip fabric back other way, over seam, so right sides of fabric are facing (a).
Stitch another seam with a slightly larger seam allowance (3⁄8 inch) to enclose first seam (b). Flip fabric open, and press seam to one side. On right side of fabric, topstitch along pressed seam (c).