What is the best way to learn content for history?

There is not a signular approach towards learning the subject that suits everyone, the best way to find the most suitable technique for you is to try a couple of different ones. The ones that have proved very beneficial for me are flashcards and mindmaps, used together.
For flashcards, I would recommend having information such as key dates and facts on one side of the card and on the other side having questions that matched that information. In this way you will become more engaged with the information and you'll be able to monitor your progress. After making flashcards for a topic, I would suggest getting an A3 paper and writing from memory most of what you can remember from the topic. Doing this would give you a holistic picture of what content you need to revise. I also found that having key facts written on post it notes around my house was helpful.