The only thing you need is a plugin called Backend Token.
This plugin protects your administrator folder for uninvited guests. It checks if a specific token is set in the http request. If the token in the request matches the token in the plugin parameters, the backend login is shown. Otherwise the visitor is redirected to the frontend.
1. Download the plugin from here.
2. Go to Joomla administration >> Extensions manager and install the plugin.
3. Go to Plugin manager and open the plugin Backend Token.
4. Enable the plugin and set a token.
5. Click on Save.
Now the administration login area is hidden from guests. You can login at the backend at http://www.yoursite.com/administrator/index.php?token=yourtoken.