The general format of the dd command is
dd if=/path/to/iso/filename.iso of=/dev/sdZ where /dev/sdZ is the flash device.
There is a great and serious danger ....... if you pick the wrong drive then you will wipe whatever drive you select and there will be no recovery.
So the first thing is to ensure that you can be certain the drive you are selecting is the USB flash drive.
This is a SERIOUS warning
doing the following command a few times as you insert and remove the flash drive (if you have automounting set up cancel it)
You should see the flash drive being added to the end of the output of the command when run after inserting it ..... and not being present when not inserted.
Make certain you choose the correct drive and NOT the internal drive or another external one!