Once the pins are mounted in the fingerrest, I measure their spacing very accurately and transfer the measurement to the edge of the fingerboard. Then I drill 1/16" holes.
The mounting pins will fit in these holes and will hold by friction.
I laminated a stack of celluloid to make a little mounting block, and I'll trim the end and cut off a hunk with the band saw:
Here I've threaded the block onto the fingerrest support rod before trial fitting:
I've found the only tricky part of mounting a fingerrest is getting the support rod precisely lined up so that its little standoff screws in flush to the side of the instrument. To get around this little matter, I mount the fingerrest on the fingerboard using the little pins, and screw the fingerrest support onto the side right where I want it.
Then, all I have to do is glue the little celluloid support block to the fingerrest while the whole assembly is in place:
I get a bit of Duco or other acetone type plastic solvent cement on my palette knife and squish it in between the block and the bottom of the fingerrest. Notice the big piece of low tack masking tape protecting the finish . . .
A teeny cut down C-clamp holds things in place while the glue dries:
Afterward, everything lines up perfectly:
This little trick of gluing in place is especially helpful when mounting a new fingerrest using the old support rod, which must be located in its original position.