Concentrate on the Key Idea
One of the most effective ways to do this is to start with a large circle in the center of the sheet, where you should write down the main topic idea. The trick is that this is not a name, it's not "about that". This is a description of your internal drive for the text, why you are going to write it, what kind of idea you want to convey to the reader. Sometimes, you have to think how to clearly articulate the purpose of writing.
It is important that the main idea in the text was only one! People do not perceive two thoughts, it's already a lot. Sometimes it's insulting, but you have to limit yourself. Or to divide the text into different parts with their headings and internal structure.
Develop the Topic Effectively
Around the central large circle, you should write down what you would like and can say. You should do not think much at this stage, what thought should follow,- just write out everything that comes to your mind, in an arbitrary order. Therefore, by the way, it is important not to write "in a column" - unconsciously, you will try to structure and limit yourself.
When you wrote everything out, connect similar thoughts with arrows - build connections between them. At this stage, it is clear that some thoughts are superfluous, do not correspond to the main idea or do not fit into the structure. Cross them out. Some important points may turn out to be "bare", - add examples to them. And only after that you should determine what order you should follow. When writing, of course, you should take into account that in each paragraph there must be one thought, the sentences should be simple and understandable, the main points must be highlighted, the conclusions should be powerful.