Here’s how to install an OS to a USB Drive and boot from it.

1. Download or save your OS installation in the form of an .IMG file
2. Now, you’ll use Image Writer for Windows (http://bit.ly/7gRu0n)
3. Launch the program by double clicking on file Win32DiskImager.exe and select the .IMG image file (from Step 1)
4. Select your USB drive letter from the drop down box
5. Click on Write

Once the writing process is complete, close the program and you can then boot from the USB drive.