On the main menu of Redaxo's backend find Installer and then Download new. Then search for ytils_rex_md or scroll down to find the plugin somewhere at the end of alphabetical ordered list of add-ons.
Click view to open the add-on's details. Then scroll to the bottom of the page and pick a version of your requirement (most likely the newest version).
Click download to add the add-on to your Redaxo setup.
Click Go to AddOn management. On the next page scroll to ytils_rex_md.
Click install and you are ready to use Ytils RexMd. Check Modules on the main menu. You should now see a new module with the Key Ytils RexMd Markdown editor.
RexMd is open source. It is available on GitHub.
You are welcome to contribute to the project or adapt it for your own needs. If you are familiar with Redaxo add-on development, you might want to change it for your own needs.