Download Kali Linux for Raspberry Pi image.
Use dd utility to image this file to your SD card. Assumed that the storage device is located at /dev/sdb.
Be advised that you doublecheck the location of the SD card !
This process will wipe out your SD card. If you choose the
wrong storage device, you may wipe out your computers hard disk.
root@kali:~ dd if=kali-pi.img of=/dev/sdb bs=512k
This process can take a while depending on your USB storage device speed
and image size. Once the dd operation is complete, boot up your
Rasberry Pi with the SD card plugged in. You will be able to log in to
Kali (root / toor) and startx.