A Flat Felled Seam, like French Seam is a strong seam and is usually found on denim jeans.It is really easy to do.You will need to determine the seam allowance and make the necessary adjustments.Practice on scraps of fabric first.
With wrong sides facing, sew the seams.
Cut away half of the seam allowance only on one side.
Turn to the wrong side and press.
Turn to the right side .
Fold the uncut part over to cover the cut seam and press.
Fold over and press.
Stitch along the fold about 2mm away from the edge.
Some sewing machines have a flat felled seam as an attachment. You can also use your blind-hem foot for more accurate stitching.
Hey Aysha, you know this is the opposite of how I learned to do the flat felled seam. The way I learned it, I sew it right sides facing first, and then do the cutting, pressing, folding and pressing. Then, I need to sew two additional lines of stitching so in the end, there are three rows of stitching on the seam (which are visible from the wrong side of the fabric). With your method, you only need two rows of stitching in the seam, so it's more efficient!
I actually saw your method in antique sewing machine manual.I'll try yours next time and see which one I prefer.