Just like anything in Rails 3, start by editing my Gemfile to include Rspec and Factory Girl:

group :test do
  gem "rspec-rails", ">= 2.0.0.beta.8"
  gem 'factory_girl', :git => 'git://github.com/thoughtbot/factory_girl.git', :branch => 'rails3'
end

Then following the factory_girl documentation I created a new user factory in spec/factories but got hit with an unexpected error while factory_girl was running her find_definitions step:

/spec/factories/users.rb:1: uninitialized constant User (NameError)

The solution was simple. I noticed in the generated spec_helper Rspec was loading everything nested under spec/support. So I simply moved my factories directory to spec/support/factories and I saw the sweet red dots of failing tests.

Time to go make them specs pass.