This step takes place on a host PC running Linux.
download the required disk image file
Connect a USB thumb drive to your pc
Run command:
lsblkIdentify your drive (usually /dev/sdX, X can be different for each system)
NOTE: SATA HDDs & SSDs also identify as sdX so be careful not to corrupt your data.
unmount your drive from the system
ususally will be: sudo umount
Run command:
sudo dd if=${path/to/image/file} of=/dev/sdX bs=1M status=progress; syncWait until dd finishes and you get a prompt again, safely eject the USB drive.