This second sample at the right is sewn with matching thread. Take note how the END of the FRENCH seam tapers into a CURVE !
I also used a pencil on the fabric for visibility. You would substitute a water soluble marker or thread trace your lines.
(Sometimes pencil will wash out. Test first)
I used dark thread for better visibility.
A brief summary.... French seams are sewn with the WRONG sides of the fabric together. You stitch on the RIGHT side at 3/8". Trim the seams in half. That means you will be left with a seam allowance between 1/8 and 1/4. (That's 3/16" for those of us that still think in terms of inches.) If you trim the seam allowance down to 1/8 inch it will be hard to press the seam allowance open with the iron. I don't care to burn my fingers!
Draw a line on the RIGHT side of your fabric where the French seam stops and the vent begins. The vertical line I added helps you to stitch toward the intersection. Yes, the line of stitching is taking a turn toward that point of intersection! As you approach this intersection switch your stitch length to 1.0 stitch length rather than back stitching. Back stitching tends to add too much thread. Stitching in reverse often causes a pucker on sheer fabrics.
With super pointy tiny scissors (Gingher) clip a 45 degree angle to the intersection.
Always press the first seam open before you press the fold or your seam will not be exactly on the fold.
Do NOT trim off the dog ears at the top of the vent. This tiny corner will be folded under as you fold the vent seam allowances under.
The final line of stitching on the French seam is sewn 1/4 inch. If you have a presser foot with the metal flange on the right hand side of the foot you will achieve straighter stitching lines.
As you approach the intersection where the French seam ends and the vent begins switch to a smaller stitch.
Do NOT clip off thread tails. Use a needle to thread the tail and hide the tail into the seam.
This will prevent the seam from opening.
And it looks nicer!
This is a really clean way to finish off the side seams for those light weight summer tops !