[Fire TV General] Bracket-:
Requirements:

TWRP recovery or equivalent

Root access

Android Debug Bridge (adb)

Know how to connect to device through adb (USB, network)

Ability to extract tar, lz and zip archives (7-Zip, Lzip)

Allowed apps from unknown sources through Fire OS

Ability to launch system apps (ES File Explorer, FireStarter)

USB or bluetooth mouse helps but is not required

Installation:

Install Xposed and the HDXPosed module

Download and install Open GApps ARM64, 5.1, pico using TWRP recovery

Download Google Play Store (Android TV) and rename APK to Phonesky.apk

Create a temporary folder on your Fire TV:

adb shell mkdir -p /sdcard/gtmp/lib/

Push Phonesky.apk to your Fire TV:

adb push Phonesky.apk /sdcard/gtmp/

Extract Core/gmscore-arm64.tar.lz and Core/gsflogin-all.tar.lz from the Open GApps archive obtained during step #2

Extract the contents of gmscore-arm64.tar.lz and gsflogin-all.tar.lz

Push these newly extracted files to your Fire TV:

adb push gsflogin-all/nodpi/priv-app/GoogleLoginService/GoogleLoginService.apk /sdcard/gtmp/
adb push gmscore-arm64/nodpi/priv-app/PrebuiltGmsCore/PrebuiltGmsCore.apk /sdcard/gtmp/
adb push gmscore-arm64/common/priv-app/PrebuiltGmsCore/lib/arm64/* /sdcard/gtmp/lib/

Enter adb shell and run su for root

Mount the /system partition with rw access

mount -o remount,rw /system

Create missing directories that Open GApps failed to create:

mkdir /system/priv-app/Phonesky
mkdir /system/priv-app/GoogleLoginService
mkdir -p /system/priv-app/PrebuiltGmsCore/lib/arm64

Copy the missing APKs and libraries to their proper locations:

cp /sdcard/gtmp/Phonesky.apk /system/priv-app/Phonesky/
cp /sdcard/gtmp/GoogleLoginService.apk /system/priv-app/GoogleLoginService/
cp /sdcard/gtmp/PrebuiltGmsCore.apk /system/priv-app/PrebuiltGmsCore/
cp /sdcard/gtmp/lib/* /system/priv-app/PrebuiltGmsCore/lib/arm64/

Change the permissions for the newly created files and directories:

chmod 755 /system/priv-app/{GoogleLoginService/,Phonesky/,PrebuiltGmsCore/,PrebuiltGmsCore/lib/,PrebuiltGmsCore/lib/arm64/}
chmod 644 /system/priv-app/Phonesky/Phonesky.apk
chmod 644 /system/priv-app/GoogleLoginService/GoogleLoginService.apk
chmod 644 /system/priv-app/PrebuiltGmsCore/PrebuiltGmsCore.apk
chmod 644 /system/priv-app/PrebuiltGmsCore/lib/arm64/*

Remove the temporary folder:

rm -r /sdcard/gtmp/

Exit adb shell

Download Google Play services (Android TV), variant 846 (8 for Android TV, 4 for arm64-v8a, 6 for 320 dpi) and rename APK to com.google.android.gms.apk

Download Google Account Manager 4.4.4 and rename APK to com.google.android.gsf.login_4.4.4.apk

Install Google Play services (Android TV) and Google Account Manager 4.4.4 using adb:

adb install 'com.google.android.gms.apk'
adb install -r -d 'com.google.android.gsf.login_4.4.4.apk'

Disconnect adb and reboot Fire TV

Connect to adb, enter adb shell and run su

Mount the /system partition as ro

mount -o remount,ro /system

Exit adb shell and disconnect adb

Play Store Setup:

Launch Google Play Store. Play Store is a system app and requires a program such as ES File Explorer or FireStarter to launch.

Login to your Google account. You should be able to fully navigate this version of Google Account Manager with the Fire remote, but a mouse can be helpful.

Play Store should have launched and you can now install Android TV apps and use apps that require Play Store or Google accounts.

Credits:

AFTVnews for their Play Store guide

guyHalestorm for his updated AFTVnews Play Store guide