I’ll be honest with you… french seams are my favorite seam finish ever. There are several ways to deal with inside seams, including just leaving them be, but for me, a French seam is a nice, tidy and classic finish for the inside of nearly any garment with exposed seams.
The one tricky thing about doing French seam is that you start with your pieces in what seems to be the wrong position–wrong sides together. With the right sides exposed, align the edges and stitch a heavy 1/4″ seam.
Press with a steam iron to set the seam and then trim the entire seam allowance down to 1/8″.
Trimming it keeps any threads from poking out of your seam and binds the edges to keep them from fraying.
Press the seam open, then press again, this time with right sides together as you’d normally expect to do.
Pin to keep the seam perfectly aligned align the edge.
On this example, it’s butting up to the zipper area and you’ll notice that I left that open.
Stitch at 1/4″, backstitching at the start and end of the seam. Press well, then open seam and press from the right side, pushing the seam allowance toward to the back of the garment.
For bulky fabrics like this denim, you can use a brayer to break down the fibers slightly and give it a flatter finish.