Clamp the tool and guard in your vice and start
filing the slot down to the level of the lathe tool.
It helps to have a few files with one side ground
down to the required thickness and kept for this
Finish off with a smooth file until it won't bite
into the guard material. Try to keep your filing
level - but it poses no problem if the slot flares
slightly towards the rear.
Reposition the guard horizontally in your vice with
the first lathe tool on top at the rear, clean off
any hardened glue and filings and using the ricasso
of your knife as a spacer glue the second lathe
tool in place.
Now clamp the second lathe tool and guard in your
vice and tap off the first lathe tool. File as
before finishing off with a smooth file.
Firmly clamp the knife in a set of parallels on
the ricasso and against the front face of the guard
Transfer to your drill press or drill mill and slide
the guard back to expose the rivet hole in the tang.
Line up the drill to pass freely through this hole
(preferably run the drill while doing this), lock
up the slides.