How to install Android Pie on your OnePlus 3T

Android Pie (9) was announced on 6 Aug 18, is now available as a third party Alpha build.  It works but it is a bit tricky to install

I will be listing the steps based on Nitrogen OS. My storage was encrypted from previous use of Nitrogen OS 8.1 Oreo.  This will require a massive backup job.

1. A PC and reliable MTP connection is required.
2. Upgrade TWRP https://dl.twrp.me/oneplus3/twrp-3.2.3-0-oneplus3.img.html on your current Android version first.
3. Next, if your storage is encrypted, copy out all of the internal storage, through recovery mode. Use TWRP to mount the phone to a computer as a USB.  This is faster and there are no application locked files.  The backup can take a few hours.
4. Download Android 9.0 Gapps. UnOffical Gapps   Official Gapps 
5. Download Android Pie https://sourceforge.net/projects/nitrogen-project/files/oneplus3/oneplus3_test/9.0
6. Download Magisk 17.1
7. Transfer only the Android Pie zip to the phone and flash Android Pie build.
8. Restart
9. There will be a prompt “Decryption failed” and restart in recovery.
10. At factory reset screen, format the storage
11. Restart
12. When complete reboot to recovery.
13. Copy Magisk 17.1 and flash.
14. Copy Gapps to the phone and flash.
15. Restart, check if Play Store is updating itself and Google app. After a few minutes, if Play is stuck in downloading updates, restart in Recovery and delete setup file.
    (/system/priv-app/setupwizard/setupwizard.apk)
16. Follow the wizard or manually add Wifi AP
17. Follow the wizard or in Settings, manually add Google Account
18. Enjoy Android Pie!

For Pokemon Go users, Copy the Magisk folder in Internal storage to another folder.  Make it a second tier folder.

In Magisk manager, Settings, tap on Hide Magisk.  Magisk Manager will not close.  You can now start playing Pokemon Go!  link

Edit:

19 Sep
Updated for official Google Apps and setup wizard