Sideloading Android ROM

Install the Bootloader

cd /path/to/ROM
# ensure adb and fastboot are in your path
adb reboot bootloader (or PWR and Vol - )
fastboot flash recovery PixelExperience_cepheus-12.0-20220123-1916-OFFICIAL.img
# (use physical buttons PWR and Vol + to reboot to recovery)

Factory Reset and format data

factory reset 
format data or factory reset
apply the update via ADB

Sideload and install the ROM for the phone

adb sideload evolution_cepheus-ota-sp2a.220305.013.a3-03272153.zip
# wait ages.... you can see the % in your terminal
reboot system

Enjoy!

Patching Magisk

with the ROM downloaded on your machine run:

cd /path/to/ROM
unzip evolution-ROM-date.zip boot.img

with adb installed and your phone connected to your computer run:

ad devices -l
adb push boot.img /sdcard/Download

with Magisk Manager installed open this on your phone and run:

magisk --> Install
deselect "Patch vbmeta in boot image" (there is a separate partition for vbmeta so no need to include in boot partition)
deselect "Recovery Mode" (we are patching boot.img and not recovery.img)
next
select the boot.img from Download folder

This should be successful and give you a location and filename of the patched boot.img.

Now pull this back onto your computer

adb pull /sdcard/Downloads/magisk_patched_<random_string>.img

With the boot.img now patched, boot into fastboot mode, flash the image and reboot to system:

adb reboot bootloader
fastboot flash boot magisk_patched_<random_string>.img
fastboot reboot