This method of creating a Debian Linux bootable installation USB drive uses the command dd to copy the ISO image to an empty USB stick.
Make sure the USB stick is empty and is big enough to take the Debian installation image you have selected, some of them a pretty big nowadays.
Insert the USB drive and use gparted or the command fdisk -1 to find which device is the USB drive, in this case it's /dev/sdb
In this example I've downloaded the Debian image ISO to /home/
Now run the following command (assuming the ISO image is in your home directory) to copy the image to the blank USB drive and make it bootable:
sudo dd if=debian-live-8.7.1-amd64-gnome-desktop.iso of=/dev/sdb bs=4M; sync