Follow the instructions here
https://www.qubes-os.org/doc/installation-guide/You will a USB drive 6GB or larger to copy the ISO file to (unless you are burning it to a double density DVD) and a 64GB (or larger) USB 3.0 drive to install it on.
Copy the ISO to the smaller USB using dd or Rufus as per the instructions (or burn a DVD) then boot from that. Insert the large USB into a USB3.0 port. When you get to the part of the installation process where you have to decide which partition to install it to select select the 64GB (or whatever) partition, which will be your USB 3.0 drive and follow the instructions. From my experience you often have to wipe the USB 3.0 stick first or the installer will tell you it's full.