Within this examples I assume that you install flatCMS into the Webserver root directory.
To install flatCMS unpack the flatcms packet into your Webserver directory and edit /config/new_config.php.
At least change the variable $home_url to your domain and the flatCMS-directory there.
for this domain this will be $home_url = "http://www.flatcms.org/"
Then rename /config/new_config.php to /config/config.php

In the flatCMS directory structure there are the directories /pages/deutsch and /pages/english. Beneath these directories you can add your directories which will be the headers in the menu.
Into this directories you can add your files.
Important is:
Filenames must end with .php.
Filenames must not have blanks in the filename. Blanks have to be replaced by underscores.
Files must not have HTML-headers and -footers, files consist only of the part between and tag in normal HTML-files.

Your directories and files may start with a number followed by an underscore. This number and all underscores in the filename are not part of the filename shown in the menue. In the menue underscores are replaced by blanks.

A little example is the following screenshot of www.flatCMS.org directory:

have a lot of fun with flatCMS.