I wanted to completely reformat and reinstall Yosemite on an old iMac. To do this, I needed to make something I could boot from. I got an 8gb usb flash drive and used Disk Utility to reformat it to a Mac filesystem that I called Yosemite. This disk was then mounted at /Volumes/Yosemite. I then went to the App Store and downloaded Yosemite, but cancelled the installation. This left the installer in /Applications/Install OS X Yosemite.app. Then, to make the bootable drive, I ran the following command:

yo:~ $ sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/Yosemite --applicationpath /Applications/Install\ OS\ X\ Yosemite.app --nointeraction
Password:
Erasing Disk: 0%... 10%... 20%... 30%...100%...
Copying installer files to disk...
Copy complete.
Making disk bootable...
Copying boot files...
Copy complete.
Done.
yo:~ $

I don’t remember how long it took, but I definitely had to wait a while. I’d say maybe 30 minutes or so. But it worked perfectly.