Add it to your Gemspec and require it:

# my_engine.gemspec
s.add_dependency 'itsf_backend'

# lib/my_engine.rb
require 'itsf_backend'

Continue following the instructions for “Installation in Rails Applications” in the consuming app.

Add it to your Gemfile

# Gemfile
gem 'itsf_backend'

Generate the initializer:

rails g itsf:backend:install

Add it to the initializer:

# config/initializers/001_itsf_backend.rb
config.backend_engines = %w(
  Blorgh::Backend::Engine
).map(&:constantize)

Inside the engine root execute:

rails generate itsf:backend:home_controller Blorgh::Backend --skip-namespace

Inside the engine root execute:

rails generate itsf:backend:resource Blorgh::Post --skip-namespace

This project rocks and uses MIT-LICENSE.