I think the current easiest and best way is to:

1. Install Drupal 8 with the [recommended composer template][1]

    composer create-project drupal/recommended-project my_site_name_dir

2. Add CiviCRM:

    cd my_site_name_dir    
    composer require civicrm/civicrm-asset-plugin:^5.25 civicrm/civicrm-drupal-8:^5.25 civicrm/civicrm-packages:^5.25

3. Enable CiviCRM in the modules list


  [1]: https://www.drupal.org/docs/develop/using-composer/using-composer-to-install-drupal-and-manage-dependencies