Fire up a Terminal windows and type:
sudo gem install rails
That's it.
Of course, if you're anything like me, you'll probably wonder why nothing happens. Maybe even ctrl+c / restart a couple times. Well, thing is, the installation can take an inordinate amount of time. That's just normal. If you want to make double sure your mac is actually working, just append -V to the previous command, like so:
sudo gem install rails -V
That'll print out an almighty list of everything gem install is doing, so you'll never have to wonder whether it hanged or not.
While your computer is installing what seems like the Trans Siberian Railway, grab the fattest book in your shelf and go out for a coffee*. Personally, I'm reading War and Peace. Cheers.
* You could also totally continue working on whatever it was you were working on before, but that's nowhere near as cool as lounging in a coffee shop while educating yourself and musing about what you're going to do with Ruby on Rails later.
