If you are a paying NOW Music Subscription and have a Google Chromecast device connected to the same Wifi network you are streaming from you will see the Chromecast icon appear in the bottom left of the 'full screen player'.
Click the Chromecast icon and allow the device to connect to the app, you should see and hear music playback through your connected TV device.
You can control the music playing back through your Chromecast device by using the NOW Music app on your mobile or tablet.
If you can't see the 'full screen player', press play on a track and it should pop open, if you are already playing music and don't see it press the up arrow on the 'mini player' and this will open the 'full screen player'.
To stop your casting session click the Chromecast icon in the app again and then press 'stop casting'.