Capture Additional Fields with New Universal Login

I understand that capturing additional fields at signup is now GA and there’s some documentation on it as well.

However, the documentation is super confusing and doesn’t really provide the steps needed to add additional fields. Specifically, it mentions having a Custom Page Template setup which links to a page that provides HTML. However, there’s nowhere to add that HTML.

So, if you’re able to provide a step-by-step guide on how to capture additional information at the time of signup when using New Universal Login, that would be great.