<p>I saw an extruder mod on Amazon <em>"EAONE 2 Pcs PTFE Teflon Tube (2 Meters) with 4 Pcs PC4-M6 Fittings for 3D Printer 1.75mm Filament (2.0mm ID/4.0mm OD)"</em>
Anybody know how this is fitted?  Is it simply tapping the feed hole on the top?</p>

Is
this
what you are referring to?
If yes, the cold end of the extruder is nomally already tapped and you simply have to screw the new fitting in it.  The PTFE tube itself needs just to be fed through the hole in the fitting
until it cannot go any further
.
Failing to do so will most likely result in a clog and/or leaking.
It is a self-locking mechanism, in order to release the tube you have to press
down
the plastic flange on top of the fitting while pulling
up
the PTFE tube.