I filled the fret slots with 0.024"(0.6mm)thick veneer
strips. These were so fragile that I had to stick them into the
slots dry before applying a thin, liquid kind of CA glue (super
glue) which flowed easily into the gaps on either side of the
strips. Use wood glue for thicker types of veneer which can be
pressed into the fret slots with the help of a clamp.
Leave it all to dry for several hours; then take a chisel and cut
the veneer strips flush to the fretboard surface. Take care not to
mar the latter with the sharp edges of the chisel; better let the
veneer protrude just a little and then finish the job with
sandpaper.