I’m using a workflow from Apple’s KB article HT201372. It works really well and doesn’t require any 3rd party scripts. You will need to download the installer for either Yosemite or Mavericks from the AppStore. If you have already installed yours you can download another copy by looking in your Purchases. If it is currently on your HD, you can dump it and install the latest by going to Macintosh HD and looking in the /Applications folder. This is different than your user ~/Applications folder.
Where it says, “/Volumes/MyVolume” you will want to replace with the path to your USB drive. For example, mine was /Volumes/Yosemite – then hit return. You should see the following on the Terminal screen.