To use the Hacker News API with Ruby on Rails, start by creating an instance of the client and initializing it with your API key. Then, create a new controller to handle requests for top stories and comments. Use pagination to limit the number of items returned from the API. To display a single story, render a partial template that includes the story details and links to related data such as comments. Finally, to dive deeper into each thread, create a separate controller to handle requests for individual comments and render a partial template that displays the comment text along with its responses.