There are three tipes of brainstorming: making a list, mapping and freewritting.
When you make a list you just write words, sentences that have a conexion whit your topic.
Mapping is another way os brainstorming, you write your topic in the middle of a sheet and you start to write ideas around it inside of circles.
And the last one is freewriting, in my opinion the easiest one, because here you write whatever comes into your mind no matter if you have mistakes because you can cross it out.