Mindmapping, or using "radiant" diagrams to outline ideas, projects and study materials may be so commonplace nowadays that people view it with a benign yawn.
But Tony Buzan was one of the first published writers to describe how to use this free-form method to brainstorm ideas, and he is still the best authority.
If you were given to drawing graphics in the margins of your notebooks you'll want to use his suggestion to make little cartoons to represent concepts.
But for people like me, who can't draw a convincing bird, you may want to simply start in the middle of a piece of paper with your topic in a circle — then draw lines outward and label each with a subtopic. From each branch you can branch off more and more — one thing I've done over the years is sketch out a simple mindmap for an article, then redraw it several times, refining it more and more.
One of the things I do is use different color pens, or markers on white boards, in order to keep ideas distinct.
No matter how you do it, mindmapping is an excellent tool — especially for tackling big projects, or things you may be stuck on.