I assume you already have ruby 1.9 and rubygems.
Terminal.app on OS X, Command prompt on Windows and a myriad of application on Linux.
Download it from http://www.mongodb.org/display/DOCS/Quickstart.
Run it from a terminal with
Download git from http://git-scm.com/ and install it.
Go to a directory of your choice (inside it we will create a new directory for D&D on Rails).
git clone https://firstname.lastname@example.org/vrinek/Dungeons-and-Dragons-on-Rails.git
to download the application to a new directory named Dungeons-and-Dragons-on-Rails.
Install bundler with
gem install bundler.
rbenv rehash to update the executables (optional for rbenv).
bundle install to get all gems necessary to run the application.
bundle exec rspec spec to make sure everything works fine (green is good, red is bad).
Start the server with
bundle exec rails server.