A french seam is a useful skill to know for people who sew. A French is useful when you do not want your seams to have raw edges on the inside, such as when you’re making clothing or bags. It gives a nice finished edge. Just make sure to have an iron ready to press your seams.
2. Lay the 2 pieces wrong sides together.
3. Stitch a 1/4″ seam
4. Trim extra down to 1/8″
5. Fold back upon itself, over the seam, right sides together
6. Sew another 1/4″ seam, covering the previous seam inside the pocket
7. Press flap to one side. You could even do a top stitch to stitch the flap down.
To learn more about general sewing, such as notions, fabric types, machine tools, and more, check out this book