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
_plugins/ in the root directory of your
For an absolute
"Hello, World" we can do:
which prints out to the terminal when you
bundle exec jekyll serve. Look for it