Measure the distance between the nut and the saddle (scale length). At this point, you can research how to calculate fret spacing (if you are a nerd. actually, the math is really easy, but its already out on the web in a million places), or you can enter this distance into a fret calculator. I use buildyourguitar.com/resources/fretcalc/fretcalc.htm
. You'll have 12 frets... the program will ask you for that.
Mark the locations given by the calculator on both sides of the neck.
Install the cotter pins at each of these locations. The cotter pin will poke through each of the layers of corrugation in the fingerboard, so it will take some light pressure and some patience. If the pins do not lay flat, you may need to use some wood glue to hold them against the finger board. If you are going to use glue, be sure to double check your fret locations, you won't get another chance.