The French seam is an enclosed seam (or finishing seam) as it does not require further finishing. Because this type of seam is bulkier, it is best for sheers and if the finished seam is 1/4″ or less. It is most definitely not for the cotton that I used below, but I wanted to try the seam out.
1. Place Wrong Sides together
2. Figure out your Seam Allowance- Okay, this is where it becomes a bit tricky. Most clothing patterns use a seam allowance of 1/2″ to 5/8″. So say your seam allowance is 5/8″ and you want to sew a French Seam, you will first stitch a line 3/8″ from the edge and then trim it to 1/8″. Notice that I have trimmed just the black floral fabric to demonstrate how much to trim. The second picture illustrates how I trimmed the entire seam. The math will make more sense in a little bit.
3. Pressing- Press open your seam and then fold your fabric right sides together and press again. The stitched line will be exactly on the edge of the fold as seen in the second picture below.
4. Stitch Again- the second stitched seam will now be 1/4″ from the fold and then press to one side (in my case up towards the bodice). In the third image, I am “holding” the French seam away from the fabric so 1) I can admire it and 2) to show you how it looks and why it is considered a finishing seam. The last picture is of the front of the piece pressed and completed seam.
**So now, let me clarify the math as concisely and as easily as I can***
For 5/8″ Seam Allowance
1.) First seam is with WRONG sides together using 3/8″, trimmed to 1/8″, pressed open
2) Fold fabric so RIGHT sides are together (press) and stitch second seam 1/4″ from fold
So if you take 3/8″ and add it to 1/4″ (think of it as 2/8″) then you will see that it adds up 3+2/8= 5/8″. So your seam allowance is still 5/8″ total.
For 1/2″ Seam Allowance
1.) First seam is with WRONG sides together using 1/4″ seam allowance, trimmed to 1/8″, pressed open
2.) Fold fabric so RIGHT sides are together (press) and stitch second seam as 1/4″ from fold
So if you take 1/4″ from the first seam and add it to 1/4″ from the second seam you get 1+1 / 4= 2/4″=1/2″
Happy sewing! I hope this helped!