The most important thing for me to really cement something is producing (vs consuming). Reading docs, watching videos... those are consuming. Of course you do have to do these things to get started. When most hackers thing of producing they probably think of building something - meaning coding a small project - but for me, to really really learn something, absolutely nothing comes close to teaching it to someone else.

So when I want to learn something really well, I prepare to teach a course or give a lecture. Sometimes there is some synergy between what I'm working on and what other junior engineers on the team are working on, and if they're amenable I'll try to teach them what I know. But even if I don't have an audience, I'll tackle a tough concept by prepping to give a lecture on it. That includes practicing in front of the mirror, or doing live coding, and speaking out loud to myself.

I haven't done any blogging but writing (after the fact, not while you're consuming the course or docs or book! forcing yourself to recall is essential to cementing into long term memory) can also be helpful.

Let's say I'm doing a course on async Javascript and the instructor explains how promise.all() works. Instead of a bunch of bullet points just regurgitating the course, my notes will just say "TODO: Implement promise.all()". Then I go and do that a day or two later. This creates a surprisingly long lasting memory, for me at least.