Download TWRP Open Recovery for kebab
eu.dl.twrp.me
When you are on the latest firmware, then flash the recovery image in bootloader (fastboot flash recovery recovery.img).
format data in twrp,
reboot recovery,
flash rom then twrp-installer,
reboot recovery,
flash gapps then twrp installer,
reboot recovery,
flash magisk then twrp installer,
reboot system.
done.
When you dont know how to get the latest fimrware on both slots, then look in the crdroid 8.x thread,
there i have postet a datailed process.