A few days ago I started a new project, and thought why not go with the latest and greatest, Rails 4 and Ruby 2.
Surprisingly I had to copy-paste snippets from several different places in order to get everything up and running.
In an effort to make that time go to good use, I put up a simple 3 page guide showing the steps I had to take to get it to work, just in case anyone else needs it.

You can find the guide here.

Edit: Well, the guys at Heroku are fast! Their tutorial now contains most of the pitfalls I talk about in my guide and will work for everyone that already knows how to get Ruby 2 on their computer.

Let me know if you found it helpful!