The simpest thing is to use ohnonot's method described in post #2,
but double-check that you will be writing to the correct drive. If the internal drive is /dev/sda, and there is no other drive except the USB drive, that drive is /dev/sdb. If you have also other drives connected, it might be /dev/sdc, dev/sdd etc. So it is important that you identify the drive you want to use. If you write to another drive, that drive will be overwritten and its content will be lost.
sudo dd if=/path/to/your/ubuntu.iso of=/dev/sdX
Still, that single line suggested by ohnonot is the simplest method available. One single command. It cannot be simpler. You need no pre-formatting. The only thing is that you have to write to the correct drive. If you are confused by my script, that is intended to help you avoid writing to the wrong drive, don't use it. Check manually, make sure there is no typing error and run that dd command line.
Ask and ask again, until you are confident that you know which device letter is used for the USB drive, that you want to make into a live USB drive. For example, do you know which commands that will help you identify it?