The drupalauth4ssp module makes it possible for Drupal users log into a SimpleSAMLphp SAML identity provider configured on the same virtual host as the Drupal site. It provides a tightly integrated login experience, sending unauthenticated users to the Drupal login page to log in. As a result it removes the requirement to produce a theme for SimpleSAMLphp since the end-user never seems any of the SimpleSAMLphp pages.
You must have SimpleSAMLphp installed and configured as a working identity provider (IdP).
For more information on installing and configuring SimpleSAMLphp as an IdP visit: http://www.simplesamlphp.org
Assuming the prerequisites have been met, composer require 'drupal/drupalauth4ssp:^1.0'.
The configuration of the module is fairly straight forward - you will need to know the name of the authentication source that uses the drupalauth:External class, this is in (<simplesamlphp_path>/config/authsources.php).