Download image from http://cdimage.ubuntu.com/releases/20.04/release/
curl http://cdimage.ubuntu.com/releases/20.04/release/ubuntu-20.04.5-preinstalled-server-arm64+raspi.img.xz -o raspi.img.xz
Copy to drive
xzcat raspi.img.xz | dd of=/dev/sdX status=progress
Install Raspberry Pi Imager.
Select model and OS (Ubuntu 20.04 arm64)
Move files in the cloud-init directory to the root of the flashed drive.