First, have MySQL and gcc installed.
Then, open the Terminal and use this command:
sudo curl http://students.washington.edu/kgc/ror.sh|bash
After a few minutes you should see something like this:
Select which gem to install for your platform
1. mysql 2.7.1 (mswin32)
2. mysql 2.7 (ruby)
3. mysql 2.6 (ruby)
4. mysql 2.5.1 (ruby)
5. Cancel installation
Select the option highest on the list that has (ruby) in the description. In this case it's 2.
Here's what the script installs:
readline 5.2
ruby 1.8.5
rubygems 0.9.0
rails
fastcgi 2.4.0
ruby-fcgi 0.8.7
pcre 6.7
lighttpd 1.4.11
mysql native bindings
If you have any problems, leave a comment and I'll see what I can do.
If this helped you, please Digg it. That way it's more likely to help others as well.
The script is an updated version of the process found here.