This is a connector type plugin handling login through CAS.
You can follow the official documentation to install this plugin.
answer build --with github.com/nizil/answer-plugin-casIn the Admin panel of Answer, within the Plugins menu, enable CAS Connector.
Then, from the CAS Connector configuration page, you must fill the Server URL entry with the URL of your CAS server.
Optionaly, you can provide a Display Name to personalize the login button (default to CAS).
Finaly, you may want to disable new registration and password login from the Advanced/Login panel, hence forcing your users to use the CAS login.
Simply login using the new Connect with <Display Name> button.