I have posted this site in past.
http://www.pendrivelinux.com/
Here is a step by step to create a usb flash drive with Linux that you can boot from.
It is handy to have a spare os loaded with your customized desktop to take anywhere you go or to just use for home purposes.
Sort of your personal mini computer ( jk ! ) on a key chain

( as long as you have a computer to boot it !)
Insert a usb flash stick in the computer ( make sure there is enough GB, try at least 4 - check to see if it is formatted FAT32)
Now...
First, and this is so simple now, download the universal usb installer here
http://www.pendrivelinux.com/universal-usb-installer-easy-as-1-2-3/
Download a version of Linux that you want to use... here are some
http://www.linuxmint.com/
See attached image to start...
From the universal installer-
Select a version of Linux from the drop list in step one.
Step two: open browse and find the Linux iso file you had just downloaded and select it.
Step 3 select the usb letter ( usb flash stick is installed in an available usb slot on the computer )
Finally click create.