The key to learning is to recognize that it takes time, and no magic method will fix that.
The point of writing it out is to force you to put it in your own words. Often times we feel like we understand something, it seems to make sense when it’s right in front of us, but when we go to explain that knowledge or to apply it we discover we don’t really get it. You can only put something in your own words if you really get it.
Different disciplines really need different techniques. When I took really hard history classes in college, I would outline the chapters–often from several books, some assigned, some supplemental–outline the lectures, and then reread both of them until I could unify them into one super-outline. By the time I wrote the super-outline down, I didn’t just know the facts, I saw how it all connected together because outlining demands that. I rarely looked at the super-outline after: it was the process, not the product, that mattered.
But as a teacher, I find that technique really doesn’t apply to either of the courses I teach, AP Economics or AP English Language (which is a writing class).
For both of these classes, you have to be able to do things. For writing, it’s a matter of writing, re-reading your own writing critically (this is the hardest, and most important, step) discussing what you wrote with a professional (that’s me), and then re-writing. You don’t have to go through that whole process with everything, but the more you do it the better off you are.
In Economics, you have to be able to apply knowledge. It’s all about reading the chapters and working the example problems (not just reading where the chapter works them out) and then working all the problems at the end, then checking your answers and staying with it until you’ve figured out why you were wrong when you were wrong. Outlining econ would be a waste of time: you need to spend all the time on practice problems and, again, figuring out why you were wrong. I suspect music theory is more like this, but I don’t really know.
For rote memorization (of which there is some even in higher-level stuff), nothing beats flashcards. Make up a big stack and go through them, making a “Know” pile, a “don’t know” pile. Go through the “don’t know pile” and repeat process until they are all in the know pile. Repeat with the whole “know” pile as many times as it takes to get to where you can go through the whole thing without missing one.