If you've already been using other services on your Sonos, then using Plex should feel quite familiar. You'll be able to choose a music library on a Plex Media Server, browse and play content within the library, and even switch to a different server if you have access to more than one.
Choosing a Library
Once you have a Plex Media Server selected, you'll see any available music libraries from that server. This will include Premium as well as standard music libraries.
Select the desired music library so you can access content within.
Browsing Within a Library
Once you've selected a music library, you can access content in multiple ways. You can shuffle and play the entire library, browse by artist or album, see your playlists, and even help get ideas for what you might want to listen to via Discover.
When browsing the library, you can start playback or perform other actions on artists, albums, individual tracks, or even playlists.
Related Page: Sonos > Playback
You can choose to shuffle the entire contents of your library for playback. The playback will basically behave like a "radio" from other Sonos apps.
Access any of your music playlists from the selected Plex Media Server and then you can play the entire playlist or a particular track.
Browse by artist and then dive into specific albums and tracks for any particular artist.
Browsing by album is similar except that there's no Artist level to it.
The Discover view from a server will let you quickly see recently added content. It also lets you see other content such as recently played artists, more content by a particular artist, other content from a genre, most played content from a particular month, and more!
Discover is designed to help you quickly find content you might want to play.
Interacting With an Item
While you're browsing your library, you'll see playlists, artists, albums, tracks, etc. You can interact with almost any of those. The method can depend on the platform you're using to control your Sonos (e.g. mobile app vs a desktop app), but it should be familiar to you from other Sonos services. For instance, in a mobile app, you can long-press on an item.
When you're interacting with an item, you'll initially see standard Sonos actions allowing you to play immediately or manipulate the Queue on your Sonos device.
So, if you chose Play Now for an artist, then it would immediately play the content you have for that artist. Things work similarly for albums, playlists, etc.
You can also choose a More... or an Info & Options type item to access additional options. These will let you do things such as read an artist's biography, read an album review, or start a Plex Mix.
Selecting a Different Server or Library
If you have more than one Plex Media Server of your own or you have some friends or family sharing with you, you might have more than one server. In order to switch to a different server, first choose the Other Libraries option from the main page.
You'll then be presented with a list of sources (Plex Media Servers) available to your account. Choose the desired one and you can start browsing its content.