This method of getting a live install to a USB drive is the simplest available using Unetbootin. Note that this will format the USB drive and erase its contents.
1. Plug in your USB Drive (Minimum USB Drive capacity 4 GB)
2. Format the USB drive to FAT32
3. Download Unetbootin from http://unetbootin.sourceforge.net/
4. Start Unetbootin and select diskimage
5. Select your USB drive
6. Then select the OS from the list or browse to the disk image
7. Now click “OK” to create a bootable USB drive