Using Scrapy with authenticated (logged in) user session

In the Scrapy docs, there is the following example to illustrate how to use an authenticated session in Scrapy:

class LoginSpider(BaseSpider):
    name = 'example.com'
    start_urls = ['http://www.example.com/users/login.php']

    def parse(self, response):
        return [FormRequest.from_response(response,
                    formdata={'username': 'john', 'password': 'secret'},
                    callback=self.after_login)]

    def after_login(self, response):
        # check login succeed before going on
        if "authentication failed" in response.body:
            self.log("Login failed", level=log.ERROR)
            return

        # continue scraping with authenticated session...

I've got that working, and it's fine. But my question is: What do you have to do to continue scraping with authenticated session, as they say in the last line's comment?