When users opt-in to use Flow on their user talk page in Beta features, we'll show them a three-step guided tour, to explain the new system.
Pau's prototype has the model.
Title: Welcome to your new user talk page
Text: Your talk page is the place where other editors can contact you. With Flow, it's easier to start new topics and keep track of conversations.
Title: Find your old conversations
Text: Previous conversations have been moved to an archive page.
Title: Give us your feedback!
Text: You can get more information, provide feedback, or disable the new system any time from the Beta features section.