1. Download ckeditor module from the url https://www.drupal.org/project/ckeditor
2. unzip the downloaded zip file. Then copy ckeditor folder (which are extract from the zip file) and paste into your_drupal_site_path/sites/all/modules
3. Now we will open modules section from Administration > Modules. Select CKEditor module check box to active the module then click on “save configuration” button to apply changes.
4. Clear all caches from selecting Home >> Administration >> Configuration >> Development
5. Now ckeditor is successfully install in your drupal cms ,just open your content and edit your page. now ckeditor will be shown instead of textarea.
if you used ubuntu os and installed drush then just type following two command to intall this module.
drush dl ckeditor drush en ckeditor