Easy as pie.
http://www.youtube.com/watch?v=ab8GtuPdrUQ
"All you have to do is go to password settings and click the button"
And you only have to go there once ever.
I think it's usually assumed users can navigate menus, because even if they can't there's not much you can do to help them at this point.
It's right there in the security tab. TWO clicks.
1. open preferences
2. click on security
And it's RIGHT THERE. That's about as obvious as I can imagine it.
http://www.youtube.com/watch?v=ab8GtuPdrUQ