1st Make sure you have the no-verity zip in the op on your external drive.I downloaded this mod: androidfilehost .com/?fid=11410932744536996371 and flashed it, works fine, but TWRP (3.2.3-1) doesn't work. unable to mount storage it says, tried changing the file system type to EXT2 and back to EXT4, it didn't work. also, MTP doesn't work, so even when I Format Data completely, I can't put anything to flash it. can't even flash a working TWRP, what do?
Then if it boots to twrp after installation of firmware, go to wipe and then wipe data. Type yes and press the check mark. Go back to main screen of twrp and to reboot and do this for recovery. Then mount system and all and then go to install and install the no-verity zip. Then reboot system. After setting up you can install magisk if you want root... good luck.