One approach to brainstorming is to make a numbered list of ideas. Simply list the ideas as they come to you and give them numbers in that order, without any attempt at organizing them. Here are some examples from my own work. I want to write about the composer Morton Feldman. I have no idea what I want to say, but I know that I have something to say.
Once you have a list of 20-30 ideas, then you can put them in some kind of order, grouping them thematically, arranging them in more logical sequences or by order of importance, discarding the redundant ones.
Of course, you could jot down ideas without the numbers, but the numbers help me to see them as discrete items rather than as a a chaotic mess. You are still allowing ideas to flow freely, but you are using a very simple technique to keep track of them. Some ideas will end up being major points, others very minor ones. Some will be discarded later, but none of this effort will be truly wasted.