How to enable Google Assistant on your phone Android 7.0+ only

Requirements

Android 7.0 or higher
SuperSU or kernel with dm-verity disabled
Latest Google app (6.5.35.21 or newer)
build.prop tweaks

Method 1: Manual editing
Just add or replace these lines on your build.prop:

Code:

ro.product.model=Pixel XL
ro.opa.eligible_device=true

Method 2: Flashable ZIP by @FaserF
Link: https://www.androidfilehost.com/?fid=385035244224384004
Flashing video guide by @mjp93

What languages are supported by Google Assistant?
Only English and German for now.

I'm still having Google Now, what can I do?
Go to the Google app info and wipe its data.

Now my phone model is Pixel or Pixel XL, is this a problem?
Maybe. Some manufacturers uses the build.prop data to offer their apps and updates on Google Play, so you can have problems updating those apps. Nexus owners won't have any problem.

My camera is crashing!
Restore your build.prop and use the new method.

What about Android Marshmallow?