Click on any of these images for a closer look.
First find the exact position you want, then put a couple drops of "crazy glue" through the screw holes. In a couple minutes, the bases will be held frimly enough to make a nice scribe line.
A sharp twist will remove the swivel, you now have that line as a guide. I use a small router to deepen the mortise.
Inletted to final depth...now drill for the screws and install, be sure to bring the slots n/s.
Shape/polish to match contour of stock.