French seams are a pretty way to finish off your seams if you have delicate, sheer, slippery or prone-to-fraying fabric because the raw edge ends up being enclosed in the seam. I used french seams in my Airelle Blouse because I was using a light weight, semi-sheer voile. They give a lovely professional finish without bulk and they are really simple to incorporate into your garment.
The trick to using French Seams is figuring out your seam allowances before you cut your fashion fabric. You could do this anyway you saw fit, as long as your first seam is smaller (or the same size but can be trimmed down) as your second seam, you should be fine.
For reference, the Airelle Blouse comes with a standard 5/8" seam allowance. For my side seams, when cutting out my fashion fabric, I decided to add an extra 1/8" to my side seams so I had a total of 6/8" seam allowance. Keep reading for more explanation of why I did this in picture form...
1. Rather counter-intuitively, pin your garment wrong sides together.
2. Sew your first seam together. I sewed mine at 3/8" seam allowance (half of my seam allowance of 6/8").
3. Trim your seam allowance down (I trimmed mine by about 1/8"leaving 2/8") and press your seams open.
4. Turn your garment inside out and press your seam flat. Pin in place and sew again at the remaining 3/8".
5. You now have a lovely, perfectly enclosed 3/8" seam.
You don't have to use my measurements for your French Seam though. For example, I cut the standard 5/8" seam allowance for my sleeve seam. I made my first seam 1/8" and then my second seam 4/8", ending up with a larger 4/8" finished seam.
Choosing how much to add to your seam allowance (if any) really depends on how much of a finished seam you want left at the end!
I hope this was helpful, please let me know if you have any questions in the comments...