The system extension is part of common Composer distributions such as typo3/cms-base-distribution. It is activated by default and ships with a default configuration for editors.

If you don't use any distribution the system extension can be installed via:

composer require typo3/cms-rte-ckeditor

Legacy installations

For legacy installations that do not use Composer, EXT:rte_ckeditor is already part of the distributed package.

Activate the RTE CKEditor extension in Admin Tools > Extension Manager.