What's the best way to learn something?

Learning is a constant process. One way to efficiently start learning something would be to understand the fundamentals first and dive deeper into the subtopics as you get more familiar with the core concepts. Read as much as possible and create an outline on the topic. Leverage the five Ws technique of problem solving - Who, What, When, Where and Why.

There are numerous ways to get familiarized with concepts. One way to do it would be to jot down your notes, try to make connections and try to simplify the ideas if possible. Assume that you are trying to explain this to a five year old, how would you build the topic, make it interesting either by visualizations, metaphors etc.? 

Another beneficial technique that can be of good help here, would be to put the concept into application in some form or the other. This can be done by either practicing the concepts in real life if feasible, or by observing its presence and relevance. 

Finally, try and teach the concepts to others. This is an extremely effective way to gain fluency and expertise on a subject.