Carl Heiles and Tim Robishaw have put together a guide: makeps.ps
There are a lot of things you should do to create a journal-readable encapsulated postscript file other than just call ps_open. The following routine uses postscript fonts, makes the axes thick, makes the character size large, and makes your plotting symbols or lines thick so that you can see them on a printout or for a journal. Use this instead of ps_open and ps_close. These functions are simply wrappers around those functions (they default to color – the keyword nocolor makes b/w only). If you want to send other keywords, the wrappers will pass them along.