Clone the Ws Auth onto to the corresponding third_party structure:
git clone https://github.com/ahmetson/CodeIgniter-Ws-Auth.git ws-auth
Then in your controller, example Auth.php add the package path and load the library like normal
$this->load->add_package_path(APPPATH.'third_party/ws-auth/src/ws-auth');
$this->load->library('ws_auth');
$this->load->remove_package_path(APPPATH.'third_party/ws-auth/src/ws-auth');
Once you enabled the Library, go to the IoN documentation, to read for avaialable methods. But in your scripts, call the methods by using ws_auth instead of ion_auth library.
Add the tables from /sql/ onto your database. Those are authorization related tables.
Then, add the sessions table to track the Sessions.
The more information and the scheme of the table could be found on the documentation of Symfony Sessions PDO Handler