1. The first step is downloading the OS you can do this by going here:http://www.kali.org/ . Next you should find out what bit your computer is 32 or 64 bit you can do this by going to computer and clicking system properties. Then you should download the latest version of backtrack that is your bit type.
2. After it downloads you should get a program called UNetbootin which can be downloaded from here:http://unetbootin.sourceforge.net/, once that is downloaded you should run UNetbootin and select the OS you downloaded, you should have a blank USB to install the OS on so plug it in and let the program install the OS.