First, practice putting your fingers over your veins/arteries on the neck for five or ten seconds while standing or sitting. Get a feel for where the pressure should be by noting when you feel faint.
Then align the noose there. Put your full weight into the noose suddenly by "pretending to be dead", just relax your body and hang limp. Ideally there shouldn't be Head pressure, but there might be a little. Vision should start to get weird in ten to fifteen seconds, and you should start to get some other weird feelings as well (for me it's euphoria, bodily lightness, and a grinding staticky noise). Hopefully this helps.
Also note that it can be finicky, as you found out... so you may have to give it several tries before you get success. If you do get success, it can be really hard to pull out so just make sure you're ready to ctb.
