NOTE: this probably won't work yet! We're still rolling our first end-user release. You'll probably need to use the "RailsGame Developers" instructions below instead of this! If you're not very confident in your Rails-fu don't do it at all yet!
First, install the RailsGame gem: "sudo gem install railsgame".
Then, you'll want to create a Rails project using the template. One good way to do that is "rails -m `gem environment gemdir`/gems/railsgame*/rails/railsgame.rb myrailsgame". This uses a template to make a new RailsGame project.
You'll need to answer a couple of questions for the template. If you say to use RESTful authentication then the web site will allow people to create accounts, and by default they'll have to use them to play your game.
If you tell it to use email activation, people will need to give a valid email address and click a "confirm" link in order to make their accounts.
If you tell it to create an initial repository, it'll set up Git for all your code, check it in, and create a .gitignore file that's probably what you want. Even if you're the only one working on your game, use source code control. Git is pretty good, but use whatever you're comfortable with.
Grab the RailsGame source from GitHub. You can install it locally via "rake install_gem", like any other project that uses NewGem. The gem name will be noahgibbs-railsgame.
Now that you have the game in place, follow the instructions above to make a new RailsGame project from the template. Where it says "railsgame*" in the command line above, you'll need to use "noahgibbs-railsgame*" instead. You could also directly use the same template file from the RailsGame source directory.