You have to read a lot of it, and what you read has to be good! There's no other way!

As an example: Before you are allowed to start working on your PhD dissertation, grad students in history typically have to do what is known as a General Exam or an Oral Exam. The exam itself is usually a pro-forma sort of thing — your advisors ask you questions for an hour, and you come up with reasonably good answer to them. The preparation for the exam is what it is really about, and you read some 300 books over the course of two semesters — which works out to a couple books a day — and then meet and talk with your advisors about them. It is a grueling rite of passage, but it quickly brings you up to speed on the existing literature, and the discussion of it with an expert historian (your advisors) helps you understand how you should be processing them, calls your attention to interesting questions, and so on.

Now I'm not saying to gain strong knowledge of history you have to do that, exactly. But you do have to do something like it: read a lot and talk about it with other experts a lot. This is all still separate from becoming a serious historian (that involves learning how to research and write history, which is different that knowing what has already been written).