friction ring eg pill box or pill box cap and rubber glove on name plate ring to unscrew is normal first step needs to be 51-50 mm OD.
remove screws may need acetone and qtip.
the focus ring should lift off.
unscrew both heliciod mark entry points.
remove all damping grease, replace damping grease you should be able to reassemble the aperture mechanism by using a darning needle from mount end as shifting the screws in the mount even cooking them with soldering iron can be mission impossible.
if the heliciod is badly worn try different entry points not nice to do.
Note they are all different this is generic for nikon and you reset infinity on a star or moon
Really not a good place to start..
As david lyga suggested loose screws are far more likely. When the helical is separated, there is only one thread that will give correct focus. *******See above.