I recently made an El Grande with a custom finial and I was asked to show how I did that.
Others may have a different way of doing this but this how I do it.
The El Grande has a flat top finial and I decided to customise it by drilling/boring out a space for a finial matching the blank on the pen, bark from the Red Ironbark in this case.
To hold the finial I mounted the threaded part it screws on to in a suitably size collet, I used my metal lathe, but it could just as easily be done on the wood lathe.
The finial was then screwed on and drilled and bored out to the desired size.
Resulting in this
Part of the blank is soaked in thin CA.
On the wood lathe this time, we turn a tenon on this piece of bark so we can hold it in a collet chuck.
In the collet chuck ready for turning the finial
Turned to required diameter
Turn a "stub" to locate it in the finial
Final application of CA
part off with small parting tool (or fine saw, whatever works).....and here it is on the pen.