I had a scrap of stretchy fabric that I use as a headband, but the edges needed to be finished and it could be cut so that it had a straight line.
I first trimmed the scrap. One side had tons of fabric the other had almost none, so I made it matching and even.
Then I folded over the edges and ironed them so that it would be easier to sew. I also pinned them.
On the sewing machine I changed the tension so that it was a bit looser, and the stitches were a bit longer than normal. As I sewed, I pulled the fabric tight.
That was pretty much it. Using a straight stitch and pulling the fabric tighter turned out to be my simple fix. Happy Sewing everyone!