Cernl is pre-release software and is unstable.
Cernl requires Phalcon PHP: https://phalconphp.com/en/
Ensure your cache directory is writable by the web server.
Copy my/cl-config.php.example to my/cl-config.php.
cd my
cp cl-config.php.example cl-config.php
Edit my/cl-config.php as needed.
Copy core/themes/starter to my/themes/your_theme_name
In my/cl-config.php, change the value of THEME to match your theme directory.