The Flash Message (li3_flash_message) plugin provides a straightforward interface for displaying status messages to the user.
Add the plugin to app/config/bootstrap/libraries.php.
Libraries::add('li3_flash_message');
Make sure you have set up a default session configuration in app/config/bootstrap/session.php.
use li3_flash_message\extensions\storage\FlashMessage;
// ... some form validation logic ...
FlashMessage::write('Error, please check your inputs!');
Use the helper to output messages inside your views.
<?=$this->flashMessage->output(); ?>
In order to customize the output, copy app/libraries/li3_flash_message/views/elements/flash_message.html.php to app/views/elements/flash_message.html.php and adjust it to your needs.
For now please refer to the documented source or the test cases.