Reply by Loren

You can buy radius sanding blocks and sand
it in sections, blending the shapes. Stewart
McDonald sells those.

In factories it’s done with a jig mounted to
a belt sander. Grizzly sells those.

It can also be done with a sharp hand plane
and good judgment. You can make radius
templates out of paste board to check the
shape at various points of the fretboard.

One thing you do need for doing fretboards
is a really good straight edge. I also use
a “sanding plane” which is a piece of MDF
with plastic laminate glued to the faces to
keep it flat and sanding belt glued to one
side. I find it useful in flattening fretboards
and in leveling frets after they are put in.