I was repairing a duvet cover this week (naughty dog, Sammy!) and used a French seam for a finish that is both polished looking and durable for a piece that will receive multiple washes (again, thanks, Sammy!). A French seam only takes a little more time than a regular seam but looks much nicer. Here's how to do it:
Usually, you sew a seam by putting the right sides of the fabric together, but for a French seam, you will actually be sewing the same seam twice, so you start by pinning the wrong sides of the fabric together. Figure out what your seam allowance is and sew a seam that is half that amount. Most seams have a 1/2" allowance, so your initial seam would be 1/4".
Once you have sewn the first seam, use a pair of scissors to trim away any excess fabric so you have a small (about 1/8") strip of fabric beyond the seam.
Open up the fabric and iron this strip over to one side.
Then turn the fabric inside out so that the right sides are together. Pin along the seam if the fabric is shifting at all, and sew your second seam, using the same width as the first seam (usually, 1/4").
Turn your fabric right side out and iron along the seam to smooth out the fabric. If you want to, you can top stitch along the seam on the right side of the fabric to tack down the little flap of fabric inside. I didn't do that on this seam, but I did for a couple of other seams on the same piece.
On the inside of the fabric, the seam should look like this: a neat, finished band so you won't have to worry about the unfinished edges fraying over time.
Here's the finished look on the right side of the fabric. Without a top stitch, it looks like a regular seam:
With the top stitch, it looks like this:
Questions? Let me know!