http://www.rubyinside.com/writing-an-interpreter-in-15-minutes-with-ruby-1825.html

At the last regular London Ruby User Group meetup, James Coglan gave a talk on how to implement a Scheme interpreter in 15 minutes. He recorded a video of the coding in progress beforehand so he could focus on the narration so unfortunately the video (available in a higher resolution here from Vimeo) is without sound. There is, however, a ZIP file of HTML slides available to help you follow along.