Clustering is a process in which you take your main subject idea and draw a circle around it. You then draw lines out from the circle that connect topics that relate to the main subject in the circle. Clustering helps ensure that all aspects of the main topic are covered.
After using the brainstorm example, let’s say you decided on gardening as your topic. Your main idea of gardening would be in the center of your page circled. Anything else that you want to say about gardening you would connect to the circle with lines. You can also add more lines to extend the ideas that relate to thoughts around the circle. When finished, your clustering might look like the following: