Cast your screen from your Android device

Connect your Android device to the same Wi-Fi network as your Chromecast device.

Open the Google Home app Google Home app.

In the top left corner of the app's Home screen, tap Menu Menu >  Cast Screen / audio.

Click the "Cast Screen / audio" then Select your Chromecast device.

Cast your screen from your Apple device

Google Home Mirror is not available to for iPhone yet, to Mirror an iPhone you'll need to download a third party app.

The recommended app to use is MomoCast

After installing, Open Safari on your Apple Device, and press the open with icon 

Scroll to the More tile,

The press the 3 dots.

Enable 'Cast with MomoCast' in Safari

Once enabled press  again,

Then press  Cast with MomoCast.