johnnydoom I’ve never bothered turning a ferrule, but I think it basically involves pinching the installed ferrule with a rag soaked in solvent and spinning until you remove enough plastic to eliminate any overhang from the neck of the head.
I have never tried it but I have seen recommendations of using the back side of an old leather belt soaked in acetone and do a "shoe shine" buff of the ferrule to turn it down. The polish it off with a rag soaked with acetone to put a shine on it.
It is actually easier to order the size ferrule closest to the hosel size and not have to turn it.