It really depends on what you're trying to learn. I've found frequently zooming in and zooming out to be helpful. I first try to understand the big picture and as soon as I have even the slightest idea of how things fit together I dive into a specific technical nuance that might be vital to the whole; trying to understand and replicate (i.e., practice) something small. Once I put some pieces together on the ground level I zoom back out and repeat the process. It's like putting together a puzzle. Eventually it's possible to see which pieces go where and why, even if the picture isn't complete.
For me learning a complex topic by following different sources helps. For example, when I was starting with deep learning a subfield of AI, I would have read at least 4-5 books and several MOOCs, before I got a feeling that I understand it. It did not happen at one stretch, I gave up several times before starting to feel comfortable.
So the easiest way to understand something is not giving up, which is actually not easy. I hope there are better ways.
@viorica_vanica It depends on the reasons too. For me when I am curious about something then I overcome the willpower issues. But if it for some external reasons like passing an exam thats where it becomes quite a challange. For which I do not have a solution.