Things to prepare :After you ready and sure everything clear, the you ready to flash. Follw the guide below:
Unlocked bootloader on your Android if your phone bootloader is locked by the vendor.
ADB and Fastboot installed on your PC or MAC, if your phone is nit rooted yet.
You need to have a TWRP recovery installed on your Android device.
Have GApps package zip file suitable for your Android device. Download Google Apps (GApps)
Backup all data on your phone before flashing GApps (in case anything goes wrong). Here is a Step By Step Nandroid backup Guide for any Android Device.
First, copy the GApps package zip file on your Android device storage.
Now Boot your Android device into TWRP recovery. You may power off and press the appropriate key combination for your phone to boot into recovery mode. If you don't know how, then you can execute the following command on a Command Console or a terminal:
After your phone booted up in TWRP recovery, just select Install from TWRP home screen, then navigate to the directory where you have kept the GApps zip file and then select the ROM file.
$ adb reboot bootloader
Slide the Swipe to Confirm Flashing to start flashing proccess of the GApps.
Wait for TWRP to flash the Google Apps completely. And reebot your device after wiping dalvik/art cache by selecting reboot to system.
Your Android device is now installed with a Google Apps (GApps).
If you have any concerns or any difficulties on this guide then feel free to place a reply on this post to get helped.