The easiest way to create bootable Ubuntu USB, by using small tool named Rufus.
1. Go to Rufus website at https://rufus.akeo.ie/ '
Download the tool, install it, or you can choose portable version.
2. Use a flash-drive with a minimum size of 2 GB.
3. Run Rufus. See the picture below for the option must be set.
4. At Device column, choose your USB device target.