Official guidance: http://docs.kali.org/downloading/kali-linux-live-usb-install#linux
To create a bootable usb flash drive:
To find your flash drive’s path:
sudo fdisk -l
The example command below assumes that the ISO image you’re writing is named “kali-linux-2016.2-amd64.iso” and is in your current working directory. The blocksize parameter can be increased, and while it may speed up the operation of the dd command, it can occasionally produce unbootable USB drives, depending on your system and a lot of different factors. The recommended value, “bs=512k”, is conservative and reliable.
dd if=kali-linux-2016.2-amd64.iso of=/dev/sdb bs=512k