How to use a map on visually high-lighted text?

Again I have read :help visual-mode and :help map as in my other post
about visual-mode I can't figure out how to make it work.

Will some one enlighten me?

--
If we can but prevent the government from wasting the labours of the
people, under the pretence of taking care of them, they must become
happy. - Thomas Jefferson

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