If you want all the pieces (as in a bisected Nautilus), or if the shell is
very large, you will need some kind of saw - I don't know the details.
However, if the shell is not huge, and you just want a center slice for
display, and don't need the rest of the shell, the easiest way to do it is
with a belt sander.  Just use a coarse sandpaper to grind away the parts you
don't want.  It only takes a few minutes.  Then smooth off the two sides
with a fine sandpaper.  You should wear a particle filter (as used in spray
painting - available in hardware stores) to avoid breathing the shell dust
that is produced.
Paul M.