What is a French seam? It is a seam that has all the raw edges enclosed inside itself. Don’t worry, there are photos. It is more complex than a plain seam in that you have to do more sewing, but you use the same straight stitch (AKA running stitch). In a plain seam, and most other seams, you place the two pieces of fabric right sides together. In a French seam, you start with the pieces WRONG sides together. In this example, the seam allowance is 5/8″.
Press the seam without turning it–leave it flat and press it. Then open it out flat, so there is just one layer of fabric, and press the seam open.
Fold it so that the right sides are facing each other. Press and stitch.
Then you press this flat then open the fabric and press the seam to one side or the other. On a vertical seam, press toward the back; on a horizontal seam, press down. For the most part.
The next photo shows how it looks from the right side. You can’t tell from the right side what kind of seam it is.
You can use a smaller seam allowance if you wish, but remember to change your pattern if you have to. If you have a commercial pattern with a 5/8″ seam allowance, you don’t have to change anything. That’s all for now. Next in this series is bias tape and a bias-bound seam.