1- Download the OS X Installer (into /Applications)
2- Insert a USB stick and use "Disk Utility" to format it as "Mac OS Extended (Journaled)" and name it "USB". Use the "Erase" tab for formatting.
3- Then run the following inside of a terminal shell:
[mbp:~ brad.allison]$ sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/USB --applicationpath /Applications/Install\ OS\ X\ Yosemite.app --nointeraction
The output will look like this:Erasing Disk: 0%... 10%... 20%... 30%...100%... Copying installer files to disk... Copy complete. Making disk bootable... Copying boot files... Copy complete. Done.