19, 20, 25, 39 would all work. If you want it to look primitive, casual you can use a darker thread and deeper "bites" to mimic hand buttonhole. If you don't want the stitching to show much, you use a thread that matches what you are stitching down. Do a bunch of doodling on a practice piece and go with what you like. I am not a big fan of zig-zag or satin stitch - but it depends on the look you are going for.