You should take an exacto knife and score around the
nut where it meets the finish to break loose the finish
there, otherwise when you knock the nut out from the
side, you will take some finish with.
 
As far as laying out the spacing of a new nut, you can
get a gauge, but most pro's will measure an equal
distance in on both side of the neck and lay out the two
outer strings first. Then you measure the distance
between the two outer strings and divide by 5, which
gives you the center spacing of each string in betweet,
but this is just a reference point.
 
Moist pro's like to space the strings so that the distance
*between them* is equal (not centered), or perhaps just
a hair wider on the wound strings. If you were to just
rely on centers, the thicker strings will be closer
together than the thin ones.
 
Best advice: get the book, it's all there and check the
other post of mine in this thread for a site which has
great pics and instructions for all this.