(Bootloader needs to be unlocked first)

Boot into fastboot mode
With the device powered off, hold Volume Down + Power

Connect phone to PC with USB cable

To ensure adb will work…..
sudo fastboot devices
then reboot into recovery with TWRP
(Download TWRP if not already downloaded)
sudo fastboot boot Downloads/twrp-3.5.2_9-0-angler.img

Now (in TWRP) tap Wipe.
Now tap Format Data and continue with the formatting process. This will remove encryption and delete all files stored in the internal storage.
Return to the previous menu and tap Advanced Wipe, then select the Cache and System partitions and then Swipe to Wipe.

Sideload the ROM .zip package:

On the device, select “Advanced”, “ADB Sideload”, then swipe to begin sideload.
On the host machine, sideload the package using: adb sideload filename.zip

(Optionally): If you want to install any additional add-ons, repeat the sideload steps above for those packages in sequence.
e.g.
sudo adb sideload Downloads/lineage-18.0-20200929-UNOFFICIAL-angler.zip
sudo adb sideload Downloads/MindTheGapps-11.0.0-arm64-20210412_124247.zip

then……
adb reboot