There are only two main parts, one plastic cover and three screws you need to deal with.
Installation is straightforward and took me five minutes by following the instructions closely.
Notice that the Atlas Headrest is indistinguishable from the Herman Miller Classic Aeron Chair itself? It looks like what Herman Miller would have done if they were to have a headrest on the Aeron Chair.
I am trying to correct my awful sitting posture with this headrest by forcing myself to lean back rather than slouching forward.