I set myself a time limit and just write what comes into mind. It doesn't matter if the ideas prove to be rubbish later on (most will be!) but out of all of that will come ideas I really can use.
I've found I need to give myself permission to just write freely and kick the inner editor out of the arena of my head for a while. It pays to put this writing aside and come back and look at it later with a cold, logical head. That's where your inner editor is useful. But don't try to free write and edit together, they really are separate processes.
If you're not sure where to start on a brainstorming session, think of a character. Jot down anything about them - what they look like, what their habits are, what they think they are like, what others around them think they are like and so on. If it helps to use pictures to think of a character than do so. I tend not to do that. I think of a character who is awkward, for example, and look at reasons why they might be that way.
But whatever method works for you when it comes to starting a useful brainstorming session, stick with it. See what you come out with and have fun with this!