There are many ways to learn.
Read, watch videos, find a teacher, experiment...
Teaching is the best way to strengthen your learning.
Teach it to anyone. Teach yourself.
Teach it as though you were teaching a child.
Write it; draw it; speak it; act it out.
Identify gaps in your knowledge.
Summarise
Simplify.
Repeat.
You do not really understand something unless you can explain it to your grandmother - attributed to Albert Einstein, Richard Feynman, and others
This approach is often referred to as The Feynman Technique. You can find more detailed descriptions all over the Internet, for example here and here.
Polymath genius Richard Feynman’s memoir Surely You’re Joking Mr Feynman is wonderful book, and a great introduction to this man and his childlike passion for learning. There is also a graphic novel biography of Feynman featuring many of the anecdotes from the book.