Helium Editor Free is a lightweight editor for Joomla articles that contains both a WYSIWYG editor that produces fully valid and compatible HTML5 and a code editor that makes it easy to fine tune your HTML.
Read more: Helium Editor Free Documentation
After installing Helium Free, you must go to the Plug-In Manager and enable your new installed extension.
Read more: Enabling the editor
Depending on your Administrator template you might need to include jQuery from the Helium plugin options.
Read more: Including jQuery