I seldom use one that I have to cut a slot in, as I usually use trigger asm. However, the few I have done & when I need a thin straight slot, I have had the best results with a small thin cutting wheel on a arbor with a dremel.
I have drilled them an connected the holes & also use a small milling bit on the drill press with a adjustable milling vice, but the easiest & less time involved was with the cutoff wheel on the dremel. On guards of the knives I have made the dremel disk is too long a slot for what I usually need, or I would use it on those as well.