1. Open Google Search
2. Open Settings
3.Tap on Voice
4. Tap on “OK Google” Detection
5. Check the box for “From any screen”
You’ll have to train the voice model, which really amounts to you saying “OK Google” three times. After that, you’re good to go! Now, which commands are you going to use?
A couple of things worth noting: This works whenever the screen is on or the device is charging. In other words, you can’t wake it up with the command. Also, if you’d like, you can add the feature to your lock screen, too. Just look for the other check box below “From any screen”.
NOTE: For most Google Now-related tips, you need to be using Android 4.1 or later.