how to install ubuntu on Macbook Air using USB Stick?

reference :
+https://help.ubuntu.com/community/How%20to%20install%20Ubuntu%20on%20MacBook%20using%20USB%20Stick

+ http://www.maketecheasier.com/install-dual-boot-ubuntu-in-macbook-air/2012/08/27

Step 0:
+ http://lifehacker.com/5934942/how-to-dual-boot-linux-on-your-mac-and-take-back-your-powerhouse-apple-hardware

Step 1:

 Install rEFit (Extensible Firmware Interface )

Step 2: install unetbootin.
unetbootin-windows-583.exe

reference#
http://unetbootin.sourceforge.net/#other