User UncleFester on the iXSystems-hosted FreeNAS forum wrote a very thorough and helpful beginner’s guide to FreeNAS. With his permission, I wiki-fied it and posted it here:
https://www.familybrown.org/dokuwiki/doku.php?id=fester:intro
The guide was originally written for FreeNAS 9.10. With the release of FreeNAS 11.2, the GUI images have changed–the revised version of the guide is here:
https://www.familybrown.org/dokuwiki/doku.php?id=fester112:intro