It is fairly difficult to learn how to write non-fiction. I would suggest taking a class in journalism, but that is not truly non-fiction, just semi-fiction. It is the diet Pepsi of non-fiction. Just one calorie, not too much fiction.
Non-fiction does not come from your imagination, which is what makes it so different from poetry and fiction. Yes, you can use your imagination to embellish the truth, like in journalism, but for the most part it is either research or memory that controls the writing. I don't do much in this genre, as I prefer imagination to reality.
Authors like me use their imagination as their drug of choice. But if you want to write non-fiction, I really can't help you much. Either the words will come, or they won't. Just let it happen. Write. That's all that there is to non-fiction.