1) log in as root or open a console and substitute the user to root.
2) download Debian ISO in a temp directory.
3) identify the disk.
4) erase the disk, if needed.
5) create the bootable USB disk.
su -
cd /tmp
wget https://cdimage.debian.org/cdimage/weekly-builds/amd64/iso-cd/debian-testing-amd64-netinst.iso
lsblk --nodeps | grep -i disk
sda 8:0 0 XXX,YG 0 disk
sdb 8:16 1 7,2G 0 disk
dd if=/dev/zero of=/dev/sdb bs=1M count=1
dd if=debian-testing-amd64-netinst.iso of=/dev/sdb bs=1M status=progress