[CentOS] USB stick query

I have a working CentOS-7 machine.
I want to install grub2 on a USB stick
(without altering my present system in any way)
so that I can boot the present system from the USB stick.

Could some kind (and expert) soul explain precisely what to do.

If it were possible to boot other systems on the computer as well
that would be an added bonus.

I did google for this, but all the sites I saw
wanted to do more than I am asking.
I also tried "sudo grub2-install /dev/sdb"
but for some reason this did not do the trick.

-- 
Timothy Murphy  
gayleard /at/ eircom.net
School of Mathematics, Trinity College, Dublin