Step Flash
1) Wipe cache, dalvik-cache, data, metadata
2) Flash ROM
3) Reflash custom recovery
4) Format data (required for encrypted devices)
5) Reboot recovery
6) Format data
7) Reboot system
8) When you reach the initial settings, reboot into recovery.
9) Then install Magisk or install Akiara Kernel SU to have root access, optional.