When it comes to learning about new things, re-learning old things, and ultimatley retaining information, I've developed the following system which works great for me.
While I've never actually taken the time to learn about the process Richard Feynman used for learning things, I believe my system may be similar or at least I believe he recommended teaching the ideas to another person as I outline in the third step of my system.
I read about a topic, listen to someone talk about it or teach it, and write down key ideas. This is really just the first step where I receive the information. During this time, I try to make as many connections to other things as possible, think of analogies, and think about the why. The mantra here is, "Why is this done? Why is it done this way? Why does this information have value?"
Once I have the general gist of what is going on, I like to try to apply it in some sort of way. For example, if it's a mathematical method, I will try doing some problems or writing some code to computational do said problems. If you're a programming, writing code is a great way to learn computational methods because you need to think through all of the cases, understate what is happening under the hood, and as an experienced engineer, I find that I tend to think about what it is I'm learning in a very wholistic yet specific manner by default. If it's a new programming language, I will do some code challenges with the language. If it's a new spoken language, I will try reading a book in the language or talk to someone in it. The mantra at this stage is, "How does it work? How does it happen? How can I apply this knowledge?"
Once I've successfully applied the information or knowledge, the next step is to try and teach it to someone else. In some cases, I will write an article about it so I can, a.) explain it in my own words, b.) plus persist what I've written so I can always refer back to it later. The mantra at this stage is, "How can I communicate these ideas to another person? How can I make someone understand? How can I make someone appreciate this knowledge?"
In the last stage, I revisit. One time I read that when we learn things we really just retain it for some fairly short period of time and by returning to the material over a period of several weeks we can solidify our retention of the information. I will re-read my old posts and notes, often making new connections and adjusting the writings. I usually try and teach it to another person at this point, even if it's just a matter of explaining the high-level to them. It's especially valuable for me to need to explain it to someone who isn't familiar with the field in general as it requires me to keep things as simple as possible and not skip important parts. While I may be fairly "hand wavey" in these explanations, it's important to realize that by keeping everything as simple as possible such that I only go over the most important and fundamental parts. The win-win here is that, I only go over the most important and fundamental parts so I can truly emphasize those in my own mind and it does a good job of re-affirming my understanding and solidifying the information.