How to develop rails plugin from scratch?

Generate Plugin with command: ruby script/generate plugin hello_world
It will create file system as bellow:
- lib
    - hello_world.rb
- tasks
    - hello_world_tasks.rake
- test
    - hello_world_test.rb
- init.rb
- install.rb
- uninstall.rb
- README
- Rakefile
- MIT-LICENSE

init.rb will be executed every time when your application runs. Generally hook code is to be included here like you want to make all methods of your plugin available in your app’s models, controllers, views and helpers

Example:
#All methods in module HelloWorld will be available in all #model’s object
ActiveRecord::Base.class_eval do
    include HelloWorld
end

#All methods in module HelloWorld will be available in all controllers
ActionController::Base.class_eval do
    include HelloWorld
end

#All methods in module HelloWorld will be available in all views and all helpers
ActionView::Base.class_eval do
    include HelloWorld
end

lib/ hello_world.rb :

Example:
# All methods in this library will be available in all models, controllers, views and helpers if you write code as init.rb above

module HelloWorld
    def say_hello
        return "Hello World"
    end
    def hello_text
        return “this is text”
    end
end

Now you need to do unit test your plugin’s methods. To do so follow:
test/hello_world_test.rb:

Example:
require 'test/unit'
require File.join(File.dirname(__FILE__),'../lib/hello_world.rb')

class HelloWorldTest < Test::Unit::TestCase
    include HelloWorld # includes your library methods to test

    def test_this_plugin
        hello_world = say_hello
        assert_equal hello_world, "Hello World"
        assert_not_nil hello_world
    end

    def test_another
        assert_equal true,true
    end
end

To run these test cases, go to your plugin’s directory with command prompt and run like: ruby test/hello_world_test.rb

You are done actually with plugin!

Install.rb: It will be executed only once when the plugin is being installed.

Uninstall.rb: This will be executed when you do uninstall your plugin

README: In this file, you should write easy documentation of your plugin with examples. Also you may highlight yourself here.