Nothing needs to be done here. As long as your eZ Publish 5 is correctly configured, the REST API is available.
As explained in details in the authentication chapter of this documentation, two authentication methods are currently supported: session, and basic. The API is currently limited to one or the other. By default, basic authentication is the active mode. It requires a login / password to be sent using basic HTTP authentication. The alternative, session based authentication, uses a session cookie.
To enable session based authentication, you need to edit
app/config/security.yml, and comment out / remove the configuration block about REST