- Download, burn, and boot up the Clonezilla Live USB
- Create the filesystem image, it will be stored wherever you specify
- Reboot, again from the Clonezilla Live USB you downloaded
- This time, select to create the live CD (not USB, because you want an iso file)
- After Clonezilla is done, burn the resulting iso image to your spare USB drive
Burn the iso image to your spare USB drive: