Always work safely and make sure the mains supply is isolated before you start.
Flatten any protrusions which may obstruct the lug.
Hold the lug in position and mark the two screw positions through the lug onto the back box.
Try to find a position where you can fix both screws into solid metal rather than the side of a knockout.
Approximately 15mm apart will allow for adjustment.
Using a nail, screw or punch to make an indent before drilling. This will help to stop the drill from skidding.
Drill the fixing holes.
Fit the lug as shown. Tighten the screws just enough to stop the lug moving and then back them off a touch to allow for adjustment of the accessory.
Clean out any drilling debris.
Remember - if there is no remaining fixed lug (after the repair) then fit an earth tail from the accessory to the back box earthing terminal where necessary.