Prerequisites
- Unlock the bootloader
- Install Magisk
- Get the original boot.img of your ROM
Process
Initial Setup
- Patch the downloaded stock boot.img with Magisk
- Transfer / adb pull that patched boot.img to your PC
- Reboot to fastboot
adb reboot bootloader and temporarily boot from the patched boot.img fastboot boot /path/to/patched_boot.img
Do not flash that boot.img!!! - Wait for the device to boot (took a bit longer than usual)
- Open the Magisk app (press Cancel when asked to perform Additional Setup), choose direct install and reboot
After OTA Update
- Download and install the update
Do not reboot yet!! - Open Magisk and select Install
- Choose "Install to inactive slot (after OTA)"