Paula,
It's not written down anywhere because there isn't any one answer. Each website and template is different, so we can't specify which module positions will work best for yours, or any site.
As for the module itself, it needs to be configured to display in a real module position that would show on your page. *Most* (but not all) templates have a debug, footer or other out of the way module position that the JFBCSocialShare can be placed into. That's where some HTML will render because Joomla has to put something in a module position, but then we use some Javascript to 'pop' the share buttons out and make them float.
The big thing here is that if you switch the Position: Float to Position: Fixed parameter, it should show on your page. Once that's working, you know the module is working and you can try to find a position where it doesn't 'mess up everything else'
I hope that helps explain,
Alex