Unable to put .dmg onto flash drive

I want to dual-boot with Ubuntu and I don't have any blank CDs so I decided to put the .dmg on a flash drive to boot from that. I attempted to follow the instructions here but every time I try, some time while the file is copying to the drive, it gives me an error saying that the disk could not be read and gives me options to eject, initialize (open Disk Utility) or ignore. Then the flash drive can't seem to be used any more, and I end up having to erase it all. I've followed other guides that told me to erase the disk and partition it a certain way and one that told me to use Disk Utility to put the file on the drive, but I get errors when I try that, too, either saying that the disk is busy or there's an invalid argument. Can someone help me out? This is way more frustrating than it should be.