Insert a USB stick and open the Disks application. Next, delete all existing partitions on the flash drive by selecting each partition individually and clicking the red minus sign. Repeat this process until there are no more partitions left. Once that's done, click the menu (three dots) in the top-right corner of the window and select "Restore Disk Image". After that, a new window will open prompting you to choose your downloaded ISO image. Locate and select the file, then click "OK" to begin writing the disk image to your USB flash drive.