I wasn't quite sure what an "overstitch" was when I made mine, so I just sort of used a stitch that went back and forth from one side to the other of the seam, taking care not to make it too loose or too tight. Once I felted, the seam was invisible anyway. It worked. I don't think you can go wrong, as long as your stitching isn't too bulky, too tight, or too loose.
Anyone else know exactly what an overstitch is?