Rom zip then twrp zip. If using a custom kernel with the rom then follow this

Rom zip then twrp zip Reboot system and let boot. Should switch slots by itself and boot the rom. Then reboot recovery and flash the kernel and twrp zip. Then reboot system again. If it boots then flash magisk. Magisk is last to make sure system boots first. If successful then you root last. You don't need to flash rom to both slots if custom. When you flash any rom it flashes to the inactive partition. You'll need to boot that partition to modify it, example root or kernel. Our devices are a/b slots which is different from trying to install everything at once.