This gist shows how to create a GIF screencast using only free OS X tools: QuickTime, ffmpeg, and gifsicle.
To capture the video (filesize: 19MB), using the free "QuickTime Player" application:
Due to the high usage of this guide and the lack of comfort in Gist's commenting area, I decided to make a blog post out of this which you can find here:
http://blog.frd.mn/install-os-x-10-10-yosemite-in-virtualbox/