How to make a bootable OS X 10.10 Yosemite install drive via Macworld.
Great advice if you’re looking to avoid what I did yesterday trying to do this and ended up installing it onto my main work computer drive. Ooops. Thankfully I had a recent time machine backup and 5.5 hours later I was back up and running. Luckily I had lots of meetings yesterday afternoon so it didn’t affect anything!