This is the fret slot jig I built. Its not visible but a 1/4" peg is sticking out from under the bottom of the jig.
This is the process I used to make the 24 3/4" template that is used in the fret slot jig. The template is made of black plexiglass. The featherboards keep it stuck on the fences and table top. The router is equipped with a 1/4 foring bit.
A guide is used to position the ruler at the precise fret position. The router (a plunge router) is then reverse plunged from under the table, drilling a hole.