My fret cutting jig is essentially a sled which runs on the table saw.
The sled is nothing more than a platform made out of plywood with a miter slide that keeps the platform square to the table saw blade. On the back side of the sliding table is a pin which is mounted about 5/16″ above the table. The pin is aligned with the saw cut but it does not have to be. The reason the pin is mounted at that height is that a fretboard must fit underneath it.
Once the sled is made use it in the following way. Take your template(from LMI or Stew-Mac) and doublestick tape it to the fretboard making sure that it is aligned perfectly with the side of your squared fretboard. THE BOARD IS NOT TAPERED AT THIS STAGE. I usually use my tablesaw fence as a good, square surface to index off of.
Now you have a template with a square fretboard doublestuck to it. If you have done it right the pin on the back of the sled will fit right in the template/board’s notches. Now, to make the finished fretboard all you have to do is align the notch for the 1st, 2nd, 3rd, etc. frets with the pin and make the cuts on the table saw using the appropriate kerfing saw blade. Very simple and very accurate!