How to Work in Root Postscript Output
Use the canvas menu: Selecting menu item for postscript output creates a file whose name is that of the canvas with ".ps" appended.
Use TCanvas::SaveAs() (inherited from TPad): For a canvas pointed to by "c1", c1->SaveAs("graph.ps"); (it figures out the file type from suffix: .ps, .eps, .gif, etc.
Use the TPostScript class (several canvases): TPostScript ps("graph.ps",-111); c1->Draw(); ps.Close();