how to generate postscript file in texshop on mac

http://email.esm.psu.edu/pipermail/macosx-tex/2006-November/025541.html

as regards standard LaTeX, typesetting in TeX & Ghostscript
mode in TeXShop should produce both a dvi file and a pdf file. In
that case, TeX produces a dvi file, from which dvips produces a ps
file, from which Ghostscript (called through the ps2pdf script)
produces a pdf file. At the end of the process the ps file is erased
(unless "Save Posctscript File" is checked in TeXShop's Engine pref),
while the dvi and pdf files are kept.