If your brainstorming method involves creating a mind map starting with the thought "Project Weevil", then that's your NA: "Create a mind map with "Project Weevil" as the central thought". If the method involves dancing around in your back yard chanting, then that's your NA: "Dance around in back yard chanting".
Personally, I always put a time limit on steps that are in danger of being vague: it means that, if that's all I do, then I can cross off that NA and think about what to do next. I've moved one small step forward.
I think what I'm trying rather unsuccessfully to get across here is that brainstorming is often itself a project, albeit one that we think of as one step. So all we need to do is specify the key, the one concrete (and measurable) thing that will start the process/project, and often we'll do that and then finish the brainstorming in one go. If we don't, then at least we've moved, say, 10 minutes along the path.