Make an undercut with a small chisel. Inlet the tail first, and start by laying the plate down, inletting as you go forward toward the bolt hole. I like to file a slight taper on the edges of the plate, so the inlet gets tighter as the plate goes in deeper(or my gaps close up!).
I assume it's this kind of plate you mean?
Slot is crowded toward the lock for a right handed shooter, and parallel with the lockplate for less friction on the pull. Anal, I know.