You can find below all the appropriate instructions on how to install the CKEditor - WYSIWYG HTML editor on Drupal 7.
1. Download, extract, and place Wysiwyg module http://drupal.org/project/wysiwyg in the modules folder /sites/all/modules/ like any other Drupal non-core module.
2.Download for the latest CKEditor http://ckeditor.com/download (At this time CKEditor 3.6.6 http://ckeditor.com/release/CKEditor-3.6.6 is recommended because the latest CkEditor 4.x could not be detected from WYSIWYG API wysiwyg 7.x-2.2).
3. Extract the editor package inside /sites/all/libraries/ folder (create this folder if it doesn't exist). Make sure that the resulting structure is as follows: /sites/all/libraries/ckeditor/plugins/
4. Enable the Wysiwyg module through the Modules page.
5. Configure the Wysiwyg profiles through Home » Administration » Configuration » Content authoring » Wysiwyg profiles
6. Set the CKEditor for the Full HTML input format and Save this setting
7. Edit the Full HTML format
8. Configure the CKEditor buttons and Save your settings