How to start a minimal ruby app using Puma or Unicorn?

I have a very basic ruby example running on Thin, but I would like to know how to translate this example to use Unicorn or Puma as the HTTP server instead. Here is the code I have now:

require 'rack'

class HelloWorld
  def talk()
    return "Hello World!"
  end
end

class SomeServer
  def start(server_object, port)
    app = proc do |env|
      [ 200, {"Content-Type" => "text/html"}, [server_object.talk()] ]
    end

    Rack::Handler::Thin::run(app, :Port => port)
  end
end

SomeServer.new.start(HelloWorld.new, 3000)

This runs fine and well, but I cannot figure out how to make it run using Puma or Unicorn instead. Most online documentation I find for the two is for Rails apps. How can I utilize the multi-threading capabilities of these servers with this simple program?

use sinatra.

So to take it step by step first install sinatra and puma gems

gem install sinatra

gem install puma

then create a file myapp.rb

require 'sinatra'
configure { set :server, :puma }

get '/' do
  "Hello World!"
end

then run the file

ruby myapp.rb

by default sinatra listens on 4567 so go to localhost:4567 you can configure puma to listen on a specific port or do a lot of other things using a config file read the documentation