its not hard just find the center of the trailing edge the easiest thing to cut the slot with is a hinge cutter but you can do it with an exacto not recommended though test fit the hinges once satisfied with the fit lay the control surface up against the hinges where the surface goes in other words make sure the gap is there at the inboard and outboard edges mark the leading edge of the control surface where the hinge goes find center and cut the slot test fit and adjust the slot so that you have nice easy throws with no binding.
cover everything with monocoat and find your slot now i glue the hinge into the trailing edge with 5 min epoxy and use 8 hr to glue the control surface in dont forget to vasoline the hinge so the glue doesnt bind them and wipe off as much excess epoxy as possible
good luck