Download the latest stable (or development if you're feeling lucky) tarball from the download page.
tar -xvzf freenats-XX.YY.ZZ.tar.gz
The web folder needs to be web-accessible. If you mess with the structure (e.g. the base directory is not in the same location as the web folder) you must edit the include.php in the folder (e.g. in the web or bin) and point to the base directory.
Setup a database for FreeNATS to use (and a user etc for it to access it with), and then edit the base/config.inc.php file to include these credentials.
Navigate to http://your-freenats-uri/firstrun.php (note: you may find the file is called firstrun-.php in which case rename it to firstrun.php).
Follow the instructions in the browser to initialise the database schema and initial settings.
Afterwards rename firstrun.php to firstrun-.php to avoid script-kiddies messing with your setup.
Now setup a cron job to run the tests (do the actual monitoring!). This may vary a little depending on your system but the following is pretty Linux-common. The tester needs to run as root (or needs extensive workarounds not detailed here).
#!/bin/bash cd /opt/freenats/server/bin ./test-threaded.sh 2>&1 > /dev/null
* * * * * root run-parts /etc/cron.minute
#!/bin/bash cd /opt/freenats/server/bin ./cleanup.sh 2>&1 > /dev/null
Navigate to http://your-freenats-uri/ and off you go, happy monitoring!
Have a look at the full install wiki page.