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
https://itunes.apple.com/in/app/momocast/id931719129
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,
Cast with MomoCast.