Use an outline to study
Aside from writing I use outlines to guide my thinking while studying a subject. I write metalearning information throughout my studying process in a single note, which serves as the master note of the learning project. Often I begin with a clear topic, but sometimes all I have is a question. I use either of these as the title of my note.
As I move through the project, I populate this note with information, journal entries, links to existing notes, external notes, etc. As I do this, the note slowly becomes an outline. I could also add additional questions or problems that emerge during my study, which I could create a new outline for.
Using the outline as guide for expansion
When I already have an outline, I use it to guide me on the next steps of the research project. I reflect on what is missing in the tala/outline. I look for good questions, which are both relevant and interesting. I also look for connections between different notes. Using whatever emerges from these reflections, I expand my reading and note-taking, which naturally develops the outline. Eventually, the outline itself guides me on what to read and how to develop my own insights about the topic.