How do I create a "ps." file containing my hand written code on a mac?

You can use the Grapher application (Launchpad : Other : Grapher) to input your equations and visualize them too. Select the equation, or the graph, and then right-click, selecting copy as eps. In the Terminal, you can employ different constructs to write to ps, open the ps in TextEdit (plain text), or even Preview.

# only type the blue text

# write the eps that was copied from Grapher — from the clipboard to a postscript file

$ pbpaste -Prefer ps > foo.ps

# open the eps in TextEdit (default) as postscript

$ pbpaste -Prefer ps | open -f

# open the eps in Preview (as converted to PDF)

$ pbpaste -Prefer ps | open -f -a Preview

Here is result for the Torus Knot Grapher demo using the above technique.

Use a text editor that can create/save to plain text i.e. as a .txt file.

This can be done in TextEdit by setting: Preferences > New Document > Format to Plain Text.

When saving (or after saving) simply rename the file to end with .ps.

I would also guess that sticking to Unicode(UTF-8) or ASCII as the text encoding is the safest bet to avoid character encoding issues.

The saved .ps file should now be displayed as rendered postscript in Preview (as Preview knows how to draw the postscript instructions) and show up as postscript "source" code in TextEdit (when the .ps file is dragged onto TextEdit or opened from TextEdit).

The above solution worked fine for me using the "Hello world" example from the english Wikipedia page.