OK, I've done a few of these now, and EVERYONE with an acoustic should have a bone nut & saddle, right?
all righty, first carve (or sand) the top of the saddle to the curve you want, about 1mm taller than you want (just copy the plastic one you're replacing.)
I've just noticed that the string spacing in these pics is a hair off, its fine once things are straightened up and all is sorted.
string it up with the gauge of string you'll be using.....
using an old string, make a temp saddle and adjust it til the harmonic is the same note as the fretted 12 th fet note.
your ears may be good, but a good digital tuner is better!! get a free download like AP tuner 3.06
mark on the soundhole side of the wire, repeat for each string.
shade everything on the soundhole side
file away all the pencil marked bone. then file in from the back to make each individual string support a dome rather than a flat on top
string it up again and you're done.......
although you may want to lower the action a hair by sanding the BOTTOM of the saddle
and yes, these pics are of a doubleneck, but that doesn't affect the process.......