The USB goes in the computer, then you boot into it.
See if this helps: Time to flash is now - but how? - #16 by amarok
P.S. The luks variant download was failing recently, but it should be fixed now. See: Unable to Flash to LUKS Byzantium Image - #2 by Moon3
You might have to add --stable. See: Unable to Flash to LUKS Byzantium Image - #21 by FranklyFlawless