-Download the ROM and GApps of your choice
-Backup each and every partition from TWRP Recovery
-Make a full wipe (system, data, cache, dalvik)
-Flash AEX Package
-Install GAPPS package
-Wipe cache!
-And at last Reboot!
[First boot will take few minutes! Have patience]
Clean Flash is always recommended!
https://forum.xda-developers.com/gal...-v4-3-t3619853