I assume you've already got Sonos set up to browse and play the music via its own system and you just want to use JRiver to tag the collection properly and create playlists which can be used by Sonos?
First of all you need to import your music into the JRiver library. See
http://wiki.jriver.com/index.php/Import for detailed help, but basically what you do is pont JRiver to the network place where your music is stored.
When your files have been imported, you can browse the library in JRiver and edit the tags for individual files or in bulk. See
http://wiki.jriver.com/index.php/File_Properties_%28tags%29. The import process can actually automatically tag obvious things for you so you might not need to do any manual tagging afterwards.
You may need to "customise the views" (
http://wiki.jriver.com/index.php/View_Schemes) to be able to browse your collection in the way you want to so that you can find things easily, but the default views should get you started.
You can easily create playlists in JRiver and add tracks to them, but I'm not sure how these can be "exported" to the Sonos system. Perhaps someone with more knowledge of Sonos can help.
The other slight issue is that I'm not sure how flexible FLAC tagging is and what sort of tags you can put in these files, and whether Sonos can read them.
Is this the sort of information you're looking for and do you want anything expanding on?
Note that JRiver is a very powerful program and just using it for tagging is a bit of overkill! There may be other programs that are deidcated to tagging and therefore more straightforward to use for this purpose.
However, you could also use JRiver as a uPnP/DLNA server which, I think, Sonos is able to use insetad of its own system. In this way at least, Playlists that are set up in JRiver will definitely appear in Sonos.
PS. I'm a Cymro too!