Once everything is correct, tighten the screw. This compresses the rubber stopper so it tightens itself in the neck of the tank, but this is another place you need to be careful. Over-tightening can cause the neck to split! It might happen now, or a month from now, so don't go mid-evil on it. Just tighten it enough so you can't turn the stopper in the neck - this is something you may want to double check after a season of flying.