Download ISO image from [https://www.debian.org/distrib/netinst]. Find out the name of the USB drive using the lsblk tool. On most Linux distributions the USB flash drive will be automatically mounted when inserted. Unmount the USB partitions first.

# lsblk
# umount /dev/sdx1
# dd bs=4M if=/path/to/debian-XX.X.X-amd64-netinst.iso of=/dev/sdx status=progress oflag=sync