I’m new to the forum and just looked at those instructions. I’ve made a few thread button loops and here is what I’ve always done (I don’t have any pictures, sorry):
1) I’ve used 2 strands of a regular sewing thread in really any size needle that’s not too big for your thread. One strand of a size 12 pearl cotton would work as well.
2) Knot and anchor the thread at one end and sew through the other end of the button loop the distance of the length of the button, leaving just a little bit of slack. If I have a 14 mm button, my loop will be 14 mm long with very little slack. Pick up the other end so that you have a total of 4 strands of thread to work over.
3) Buttonhole loop over the 4 strands just as they show in the picture linked in the previous post. Just keep creating buttonhole loops, packing them in until you get to the other end of your loop. Take the thread back into your fabric where the loop ends, tie off, and you’re done.
The instructions talk about using a stiletto or pencil to keep the strands in place. I’ve never had to do that.