Joe Barry replies: You have several options to guide the router.
However, our good friends at Lee Valley/Veritas have come up with an ideal solution for doing screw slots. They now make two router bits that will rout a slot with either a flat bottom or countersink – stock numbers 16J11.60 and 16 J11.70. The newest Rockler catalog also has them now – part numbers 28441 & 21660.
These bits will rout the counterbore slot and the slot for the screw shank with the one bit. Very sweet!
Another item that should be considered is the elongated washers by Lee Valley from a design by Chris Becksvoort. They are made in two sizes for #10 and #14 round head screws. These require a 1/2″ or 5/8″ wide slot – 50K35.01 and 50K35.02.