A French seam is one that encloses the raw edges of the fabric within the seam, so there is no need to finish the edges of the seam to stop them fraying.
1 Place the fabric with wrong sides together. Stitch a seam 10mm (⅜in) away from the edge. Trim to 3mm (⅛in) and press the seam open.
2 Fold the fabric back on itself, so the right sides are together and the stitched seamline is on the edge of the fold. Press.
3 Stitch a second seamline, 6mm (¼in) away. Press the seam to one side.