Install Grub bootloader on USB

First of all you must have grub-install, if you dont have open terminal and type: sudo apt-get install grub-install
Then type sudo grub-install --root-directory=/media/your-usb-label /path-to-usb-device   (your usb must be mounted)
For example, my usb label name is PENDRIVE and path to device is /dev/sdb so: sudo grub-install --root-directory=/media/PENDRIVE /dev/sdb and wala....
then give restart and boot from usb and you will see

blablabla.............
grub>
define kernel image path with command kernel and initrd path with initrd command.. for more information type help <commandname> to get information from grub shell......