Install the Play Store. Then convert it to the system app then reboot. That's all.
To convert, you can simply use ES File Explorer. With this option, first, uninstall the Play Store. Enable root explorer in the app dashboard. Still, in the dashboard, tap local and select device. Look for system, app, and there paste the Play Store APK file there that you have copied (so you first copy the Play Store APK before going to root explorer). Next reboot.
Or you can convert to a system app using any root app like Link2SD. Here you need to install the Play Store app, next you open Link2SD and select Play Store. Tap the more options button and tap convert to system app. After it is complete, reboot.