First, you need to download the LiveCD iso image of your choice. After inserting your USB stick, you can find out what device it is
~> su # grep -Ff <(hwinfo --disk --short) <(hwinfo --usb --short)
Finally, once you’ve found your block device, write the image to it. Point ‘dd’ to the full path such as ‘/home/user/Downloads/openSUSE-12.1-KDE-LiveCD-x86_64.iso’ or change directory (example: cd ./Downloads) to where the image is contained.
# umount /dev/sdX
# dd if=/path/to/downloaded.iso of=/dev/sdX