Those bands are shims, which are selected for that specific lens to ensure that all the geometry works out. The trick to them is to place them on the lens body first, then carefully lower the mount ring onto them without disturbing their location, and then placing the screws and tightening the screws a little at a time, evenly, in a pattern like you use to tighten down lug nuts. The part will settle into the right position by itself when you do this. Never force anything. Moderate pressure on the parts, and moderate screw torque, is all you ever need. If it doesn't go together properly under moderate force, something is out of position.