But it still took the longest time before I got the hang of it, above all to understand how to actually sew in relation to the braid, so that's what I've tried to show here. This braid trick can be used on top of an existing seam, or instead of one (to close the panels of a purse, for instance), or be applied slightly further in on the fabric to decorate a collar or sleeve opening.
1. You will need fabric, a needle and scissors, some thread matching the fabric (though I should have used something with more contrast here), and two strands that make up your braid. Attach all four ends of these strands where the braid begins - you will be able to hide the ends inside a seam or tassel if you're making a bag. In this tutorial I've just knotted them and attached them randomly to my fabric swatch.
2. Put one loop inside the other.
3. Pull a little to tighten the "shed" that forms.
4. Make one stitch through your fabric. The sewing thread begins by crossing the braiding thread which you haven't lifted up, and then passes through the fabric to go under that same braiding thread. Basically you're sewing a long spiral in the direction of the braid. I should have used a sewing thread which gave a better contrast than this grey one, which I picked to match the fabric!
5. Put the next loop into the previous one.
6. Tighten the new shed.
7. Make another stitch to lock down the [red] thread.
8. Booya! I realized I should have marked the intended outlay of the braid on the fabric beforehand, by drawing or basting a line. If there's no edge to refer to, it's not altogether easy to see where you're going. Good luck!