I have downloaded and made a bootable USB of Yosemite but once I boot holding the option key and attempt to install I get an error stating that Yosemite is already installed on my computer. I'm running Snow Leopard and I made my bootable USB using Diskmaker X.
You should use following to make a bootable Yosemite USB.
sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ OS\ X\ Yosemite.app —nointeraction
Just replace the Untitled with name of the USB drive