add Devise

Gemfile
gem 'devise'
bundle install

rails g devise:install
(follow instructions...)
$EDITOR config/initializers/devise
$EDITOR config/environments/development.rb
$EDITOR config/environments/production.rb
$EDITOR app/views/layouts/application.html.haml
(notice and alert)
rails g devise User
$EDITOR db/migrate/*add_devise_to_users.rb
(change email to old_email, uncomment confirmable)
$EDITOR app/models/users.rb
(add :confirmable)
rake db:migrate
$EDITOR app/controllers/users_controller.rb
(before_filter :authenticate_user!)
(misc tidying)