Configuration Options:

https://ckeditor.com/docs/ckeditor4/latest/api/CKEDITOR_config.html

Build a simple plugin:

https://ckeditor.com/docs/ckeditor4/latest/guide/plugin_sdk_sample.html

Set Base Path

Set CKEDITOR_BASEPATH before creating the editor.

var CKEDITOR_BASEPATH = '/upload/ckeditor4/';