This is really all there is:
1. Download an Ubunto .iso from ubuntu.org
2. Plug the USB stick into your mac os x system
3. open a terminal and type: diskutil list
4. unmount your usb stick: i.e. diskutil unmountDisk /dev/disk1
5. copy the content of the iso onto your usb stick: sudo dd if=~/Downloads/ubuntu.iso of=/dev/disk1 bs=1m
6. unload your usb stick: diskutil eject /dev/disk1
7. you’re ready!
Source and more: http://www.makeuseof.com/tag/how-to-create-an-ubuntu-installation-usb-on-the-mac/