That is, use the "set mode metafile" command to output your graphics to PLT format and later (outside Ferret session) use Fprint script to produce your postscript images.
If you are using PyFerret, I think it's better to follow the instructions given here:
Paulo Santiago
2013/2/17 nitin patil
<nitinpatil85@xxxxxxxxx>
Dear ferret users,
Did any one knows how to save output plot in .ps and convert it to high resolution in publication quality.
--
Paulo Santiago
Doutorado em Meteorologia
Programa de Pós-Graduação em Meteorologia
Instituto Nacional de Pesquisas Espaciais
Tel.: (12) 3186 8637