You make a noose, using a slip knot. Your head goes in there. You tie the other end of the scarf (or rope or whatever; a scarf is OK) to something, to hang from. I'm not sure what knot you use at that end. And I think the idea is that the knot at the head end presses against your neck in such a way as to restrict the carotid artery, ideally without restricting the veins or your trachea.
NB I don't know what I'm talking about. I just thought I'd say what I think in the hope that someone will confirm it or correct whatever I've got wrong. I've read the first 2 pages and the most recent posts of the hanging megathread, and I still don't know what I'm doing.