First, if you don’t have the Google Assistant app, you can get it for free from the App Store. Once you have it, launch Google Assistant. In the Google Assistant app, tap the “visual snapshot” button in the lower-left corner of the screen. (It looks like a partial rectangle with lines radiating from it.)
On this screen, locate the section that says “Add ‘Ok Google’ To Siri” and tap the “Add to Siri” button. Sometimes this message only appears after you dismiss other notifications on this screen.
On the next page, you’ll see an overview of the custom phrase that will be added to Siri. It explains that when you say “Ok Google,” it will perform the “Hey Google” action. Tap “Add to Siri.”
After that, any time you launch Siri, say “Ok Google.” Siri will ask, “What do you want to ask Google?”
Speak your command or question, and Siri will automatically route it to the Google Assistant app. You’ll see the results when the Google Assistant app appears on the screen.
From there, you can ask again by tapping the microphone button in the Google Assistant app, or by triggering Siri and saying “Ok Google.” If you have the time, you can also set up a shortcut that lets you launch Google Assistant by tapping the back of your phone. Have fun!