After trimming and squaring off the back edges, a small tab is cut in the bottom fold. This gets clipped into a tiny swallow tail. The back seam is the same stitch as the center seam on the toe, but with a single thread. Since it starts at the bottom, it is almost never even when it gets to the top. Trim the ankle opening as required to make it look right on both sides.
Pass the thread back through some of the holes at the top to reinforce as this seam gets some strain when putting on the mocs. Time to try them on and check the fit. They will stretch some. The horizontal bulge in the picture is the fold from the previous flattening step.
The ankle flap should be slightly larger than the opening, and the width of the moc top when it is flat. Whip stitch it to the ankle opening as shown, inside the moc. Start and stop at the toe center seam. Turn the flap to the outside and DONE! Time to make another.
Trim the flap evenly on both sides, and poke holes for a lace if desired. Most period depictions show them flap up, tied over the leggings. Low-mileage footwear for the the frontier, and they feel icky when wet. A backup pair wouldn't hurt.