Assuming you would like to use Google Assistant with your phone when the home is listening:
You can say "Hey Google" to start a conversation with your Google Assistant. If you have more than one Google Assistant enabled device, make sure the device you're using is unlocked and within earshot.
If your device is locked, you can still say "Hey Google" to open the Assistant, but you'll need to unlock your device to continue the conversation.
You can also say "OK Google" to start a conversation with your Google Assistant.
To use the Google Assistant with your phone when the home is listening:
Open the Google Assistant app on your phone.
Tap the blue icon with a white mic in the center.
Say "Hey Google."
The Google Assistant will respond and you can ask it questions or give it commands.