I found Flat Stitch in both Mary Thomas's Dictionary of Embroidery Stitches and the Royal School of Needlework's Stitch Bank, where there is also a video for you to watch.
Or follow my photo tutorial.
You might ask what the difference between the Flat Stitch and Fishbone Stitch is. Well, the name gives you the answer. The Flat Stitch is flatter; i.e. the angle is more acute; the stitch has a smaller angle, is more horizontal, if you see what I mean.
You need to mark the shape you want to fill in, and place an overlapping area inside. Then you should stretch the fabric taut in a hoop.
Please note, this stitch uses a lot of thread:
Homework: Add here.

























