If you have an existing RoR application, simply create a new subdomain, with a new folder name specified, for one of your domains through the Domains page and then upload the application to that folder via FTP. When you visit the site, your RoR application should automatically execute.
If you've never used RoR before, you can create an example Ruby on Rails application by following these instructions:
1. SSH in to the webserver and run these commands from the home folder:
ruby script/generate controller hello
echo "Hello world" > app/views/hello/index.rhtml
2. Create a new subdomain through the Domains page in your control panel, for instance "hello.yourdomain.com". Point its Folder to "hello/public"
3. Visit http://hello.yourdomain.com/hello
. The rails app will automatically be detected and your "Hello world" should appear.