The process is simple. Update both your Sonos system software and Controller apps. Make sure that your iPhone (or iPad or iPod) is connected to your home network. Launch the Sonos Controller App and you should see your iTunes songs, along with your playlists and podcasts, as a music source.
Of course, your music will be played with the same Hi-Fi quality that Sonos always delivers. They've always been a powerful contender in the home audio market and this latest inclusion just further cements that status.