Read the documentation:

• Landing page for CKEditor 5 Plugin Development Docs:
    https://www.drupal.org/docs/core-modules-and-themes/core-modules/experimental-ckeditor-5/plugin-and-contrib-module-development

• Drupal CKEditor Dev Tools:
    https://www.drupal.org/project/ckeditor5_dev
  

  There's no release for this - you have to clone the code out of the GIT repository. It enables the CKEditor 5 Inspector and contains a full template for a Drupal module that implements a CKEditor 5 plugin