I am assuming that you have a sys-usb qube, and have downloaded the
installer image to a qube, QUBE_A, in /home/user/Downloads
Insert the USB stick.
Use qui-devices to attach the whole usb stick to QUBE_A.
Open a terminal in QUBE_A
Check that the device has appeared as /dev/xvdi - run dmesg and
ls /dev/xvd*
sudo dd if=PATH_TO_IMAGE of=/dev/xvdi status-=progress
Replace PATH_TO_IMAGE with the actual path to the image you downloaded.
Adjust /dev/xvdi as appropriate.
Once dd has done its work, remove the device from QUBE_A using
qui-devices , and remove from the machine.
You should now have a boot able Windows installer on the USB stick.