How to use theme switcher?

[saccori]

I'm using GetSimple 3.2.1 (latest version), i should like to allow users to switch from several themes.
I visited the Theme Switcher page (http://get-simple.info/extend/plugin/the...tcher/248/) on GetSimple Extend section but i do not understand all the details for the correct use of this plugin.
Can anyone explain me (it should be better with an example) how can i use this plugin ?

In particular i DO NOT UNDERSTAND these points:

all templates configured in any page must exist in all themes. If there is a functions.php in a theme, it must be the same in all themes Add links (or a form) to your template to allow the visitor to switch themes: To temporarily (user session) switch to a theme (here Cardinal), use a link ...?settheme&theme=Cardinal

What exactly must i do? What must i insert before ...?settheme&theme=Cardinal? What means "all templates configured in any page must exist in all themes"?
Thanks for collaboration and excuse for my incompetence...