Well… Roughly speaking the following should happen:
- Download the TWRP image for the Fairphone 3.
- Use TWRP to create a backup of the user data. As mentioned above somewhere, you can use the
fastboot boot
command to launch TWRP. - Unlock the bootloader.
- Flash an update of the LOS ROM
- Use TWRP to modify the boot partition with the Magisk stuff, basically this can be done with the install function.
- Restore user data from backup.
- Re-lock the bootloader
If anybody has some details or comments to add, go ahead.