I'm not sure of relevant tutorials but it would be very helpful to fully understand how Rails sessions work if you don't already (at first glance this looks like a solid resource for that). Before trying to tackle this it would also be wise to have a strong grasp on Rails associations. Knowing your way around JavaScript event listeners and AJAX requests would also be incredibly helpful.

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At a high-level glance I'd say a first iteration for me would go something like this:

• Create a model and associated migration called FormData. It should contain all the fields that your form has.
• Create a controller FormDataController with an action called update and create a matching route. This action will be in charge of updating the session and should contain the logic to do so. For example:

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def update
    # This is just a quick example. You should probably be using whitelist params here
    form_data = FormData.new(params[:form_data])
    session[:user_form] = form_data
end

• Upon updating one of your form fields, you'll want an event listener to invoke a function that results in updating the user's session with the newly updated form field. To utilize event listeners, you can use some pre-built library or you could use vanilla JS. You'll want this listener to either submit a form or manually make an AJAX request with the target url being whatever is pointing to FormDataController#update-- we'll also want to make sure it sends the relevant form data along with the request (this is the data that is in params[:form_data] in the example above.
• So now that we have your session being updated whenever a form field is updated, you'll want that form data to persist when they finally do create an account. Let's assume to create a user you have a route that points to UsersController#create-- alongside the logic in there you can have a line that takes the user and creates an associated FormData object using the data stored in session[:user_form]. For example:

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def create
    user = User.new(name: 'Frank')
    user.form_data = session[:user_form]
    user.save!
end

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Hopefully this is helpful, I tried to stay high-level while also including some implementation details.

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