Simple
1. umount the usb flash disk;
2. copy iso file to the device like follow command
debian@thinkpad:~/$ cp debian-12.10.0-amd64-DVD-1.iso /dev/sdx
the sdx is your usb device.
3. sync
debian@thinkpad:~/$ sync
Done!
reference link: https://www.debian.org/releases/bookworm//i386/ch04s03.en.h[……]