Maybe you are already experienced with brainstorming, or not. Brainstorming is an art that must be practiced. Brainstorming is simply to let ideas flow, without judgement, so that everything is on the table for consideration. The purpose of brainstorming is to bring from inside you the authentic and complete idea you are having. When you're done brainstorming, that's when you edit.
All too often people start to have brilliance, and then quash it quickly. The creative urge strikes, and ideas start bubbling. Then they start thinking "naw, that won't work" or "nobody will like this" or "this is nonsense", and lickety split, the ideas stop.
The core thing in brainstorming is to just write ideas. Get a large piece of paper, or a pad of paper, and a pen. Preferably gather a few people together, ones who are dedicated to the idea you wish to bring out. Then you start rambling, rolling ideas around the room and writing them down as quick as you can. Make sure to write enough of each idea so you can remember what you meant later on, but not so much that you're bogged down with the writing. The strategy is to have a smooth and rapid flow of ideas, with no stopping.
Most importantly, do not edit anything during this phase. You may come up with unrelated ideas, and just write them down and go back to the main topic. You may come up with nonsense, but don't think about that, don't try and figure out what's nonsense right now, just write. You may come up with impossibilities, and again don't worry about that, just write.