- Install this module and its dependencies (library module)
- Install the (commercial) library from http://ckeditor.com/addon/details and put it into libraries su you have a file at path libraries/ckeditor_details/plugin.js
- Go to admin/config/content/formats/manage/rich_text (or another text format) and enable "Details ckeditor button" to the active buttons.
Currently this lives on github / packagist:
composer require clever-systems/ckeditor_details
Note that the library repository is a private one and does not have a packagist entry.