Add the gem to your Gemfile:

gem 'thredded'

Add an initializer to your app: config/initializers/thredded.rb

Thredded.user_class = 'User'
Thredded.email_incoming_host = 'incoming.example.com'
Thredded.email_from = '[email protected]'
Thredded.email_outgoing_prefix = '[Thredded] '

Copy the migrations over to your parent application and migrate:

rake thredded:install:migrations db:migrate db:test:prepare

Mount the thredded engine in your routes file:

mount Thredded::Engine => '/forum'

There are a few things you need in your app to get things looking just right.

1. Add a to_s method to your user model. The following example assumes a column in my user model called name:

class User < ActiveRecord::Base
  def to_s
    name
  end
end

For more information see the full-fledged rails app