the flat plate should be on the outside of the hull. Sealant of course behind the plate. Washers and the first nut tightened up on the inside then the 2nd nut for the electrical ground wires.
I think it's an improvement to use 2 nuts on the outside. When you want to remove the anode, hold the nut nearer the anode firm with a spanner and undo the outer nut with a second spanner - this will clean off any fouling on the stud without putting undue turning force on the studs.