Here’s how the french seamed welt pockets on my current project are constructed, with bonus paper models of the process. I suspect it will be easier to see what’s going on with the paper than with fabric samples. I’ve used origami paper which has one white side and one coloured side. The coloured side represents the right side of the fabric, and the white side the wrong side.
This picture represents the right side of the jacket front with the opening for the pocket marked. When doing these in fabric I like to make the markings on the wrong side and line things up by poking pins through to the right side, but I know some people prefer to mark the right side of the fabric with something that can be removed without a trace, like basting in a contrasting thread.
Step one is to sew the welt on. It goes on the lower marking with the opening edge pointing down. The side of the welt that will be visible goes against the jacket front.
Then the front pocket bag gets placed over the top, right sides together, with the marks for the opening in the pocket bag aligned with the marks on the jacket front. The pocket grainline marking isn’t right on my model, just ignore that.
Then slash the pocket opening through both the pocket bag and jacket front, cutting diagonally into the corners. The right side of my paper pocket piece is brown, which is why brown bits are visible around the edge of the opening. The raw edge of the welt is making the slash in the jacket front difficult to see but it is there.
Turn the pocket bag to the inside through the opening and press as normal. It should look like this from the front, with the welt covering the opening.
And now the clever bit: turn the pocket and welt back to their original positions and place the back pocket bag (red paper) on top, with wrong sides of the pocket bag pieces together.
Sew around the edges, trim the seam allowances close to the seam, and turn the whole pocket back through the hole. The welt points up again and covers the opening. Then sew around the pocket bag again with right sides together, completing the french seam.
Finish as normal: sew the pairs of fabric triangles at each end of the opening together, sew the ends of the welt to the jacket front, and sew the pocket bag as close as possible to the top edge of the opening, sewing through both layers of the bag and the fabric flap from where the opening was slashed, but not the jacket front.
One thing I love about sewing is seeing how things like this get put together. It reminds me of when I was at university and learning to really think in three dimensions.