Hem the edges (top photo).

Begin sewing on the han eri: with a knotted length of thread, ensure that your needle goes through both edges. Depending on how accurately you ironed your han eri, this will be more or less straightforward:

The finished seam will look something like the mirrored photo below. It's not perfectly straight, but that's typical when you hand sew through several layers and aren't used to what's called the "stab" stitch. A running stitch is more regular, but quite difficult through so many layers (especially when one is a stiffener), so don't feel badly if your stitches are crooked - they won't be visible as long as you stay near the edge: