1. Download the desired .img file
  2. Open a Terminal (under Utilities)
  3. Run diskutil list to get the current list of devices
  4. Insert your flash media
  5. Run diskutil list again and determine the device node assigned to your flash media (e.g. /dev/disk2)
  6. Run diskutil unmountDisk /dev/diskN
  7. Execute sudo dd if=/path/to/downloaded.img of=/dev/diskN bs=1m
  8. Run diskutil eject /dev/diskN and remove your flash media when the command completes