Its easy. In the hand full of times I have done this its a breeze.
Some art work like Metallica already have the area to drill blank (no art) so drilling will not chip the art.
The apron normally comes with the two studs not connected to the apron from PBLife.
Make sure there is nothing in that area on the bottom side of the playfield.
If that is the case simply remove the plastic apron and place the metal in place, use the rivet hole to mark the area and drill a slightly larger hole than the apron stud straight through slowly.
When swapping over make sure to get the two screws that hold the apron to the two metal support brackets from PBL and a lock nut to fit each of the two metal studs for under the playfield.

I have added metal aprons with studs to the following games without issue.
Metallica Pro
Black knight SOR pro
Aerosmith Pro
GOTG pro