How to backup

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/9.5/static/continuous-archiving.html
Description:

Hello,
Could you please explain 24.3.3.3 (version 9.5) further?

Things that are not clear to me:
* What files do I need to backup?
* How to check if all files are in the expected $PGDATA directory, because I
understand that you can place them everywhere.

A short example/code would be helpful.

Thank you.
Paul (very newbee)