Generally what happens is a a publisher lets you know its available via a list of WebSub Hubs. Then as a subscriber, your client will pick one or all hubs available and send a request to the hub to get notifications about any updates. The hub then double checks with the subscriber that they'd like to see these updates and that's it until the publisher submits an update. When the publisher has an update, they poke the Hub, the hub downloads the content and shoots it out to all of the subscribers.

For the publisher, it's as easy as including

  Link: <https://hub.example.com/>; rel="hub"
  or 
  <link rel="hub" href="https://hub.example.com/">

in the header of the content.