How to make grub-mkconfig work on a usb device?

AFAIK grub-mkconfig and grub-update/update-grub commands only run on the device that they are installed on. I want to run the commands on a usb drive, but - I don't know how to install them on the usb - I can't make the ones that are installed on my computer to run on usb(i.e they should update the usb's grub.cfg file) How can I do either of them? BTW I am using manjaro KDE