There is annoying little documentation about writing a generator for Jekyll, particularly since when searching for examples, there is so much noise in form of hits for "static site generator." Two decent examples are from Ricardo Lopes and Starr Horne of Honeybadger.

Starting at the beginning, you want to be putting my_generator.rb in _plugins/ in the root directory of your Jekyll site directory.

For an absolute "Hello, World" we can do:

class Generator < Jekyll::Generator
  def generate(site)
    puts "Hello, World!"
  end
end