Well, you're not going to be a master after a week long crash course, but I bet with a lot of studying and reading, and a good tutor to answer questions and walk you through tricky parts of the code you could get to the point where you could at least read the existing code and grasp what it is doing after a week or two. From there it's just a matter of digging in and trying to solve bugs and fix stuff.

The best way to learn is to start getting stuff done. Once you can read the code you can start making changes and seeing what happens. You'll break stuff, that's OK as long as you're not making changes on the production server. You should have a local copy of the code on your computer that you can play around with without fear of screwing things up royally.

I'm not a Ruby on Rails hacker, I'm a Django/Python guy, but I'm sure there are plenty of good guides to Ruby on Rails out there that could get you started.