You need a USB drive of at least 8GB. The part that catches people is that Qubes ships an ISO which its own install instructions tell you to write as a DD image. Burn it the normal way and you get a stick that will not boot.
Download the current Qubes release ISO from the project site.
Download Rufus from its own site.
Both are plain downloads. Save them where you can find them.
https://www.qubes-os.org/downloads/
https://rufus.ie/Plug in the USB drive and run Rufus.
Select your USB drive at the top.
Set the write mode to DD Image.
This is the step the whole guide exists for. Qubes tells you to do this in its own install instructions. ISO mode produces a stick that does not boot.
Click Start.
Rufus may offer to download a syslinux component. Say yes and let it fetch.
If it asks again whether to write in DD image mode or ISO image mode, choose DD image mode.
Confirm the warning that all data on the drive is destroyed.
Wait for the write to finish.
Boot it in legacy mode rather than UEFI.
Most firmware lists the same USB drive twice in the boot menu. One entry is the UEFI path and one is legacy. Pick the legacy entry.
A stick that fails or hangs on the UEFI entry will often come straight up on the legacy one.
Partition scheme, file system and cluster size all sit above the mode selector in Rufus.
DD mode writes the image byte for byte, so it overwrites the layout those settings describe.
Do not spend time tuning them. Get DD mode and the right file selected and the rest is ignored.
Rufus has been redesigned since this was first written and the controls have moved. The two things that have not changed are the DD requirement and the file picker hiding your ISO.