I think many of us are students, or just like self-learning some topics for themselves, perhaps their work requires studying — I think it might be interesting to collect a few tips we have experience with on how to do it efficiently.

I would start with probably one of the best tools in this category that I think are still underutilized by many: spaced-repetition software. Perhaps more people might be familiar with the concept from language learning with word cards, but the base idea is that reinforcing some knowledge at increasing intervals will effectively make you remember it ’forever’. A final selling point on this topic: it only requires 10 minutes out of your entire life to remember a fact for basically forever with instant recall - that’s a very good use of one’s time in my opinion. For more information on it here is a great article.
(It’s talking about Anki, a notable spaced repetition program that is free and open-source (the ios client is paid though, as this is the only income source of the maintainer, but you could just use the web interface as well. Not affiliated))