Prewriting can take many different forms, such as brainstorming, freewriting, an idea web, and others. To explain,
Brainstorming is free association, meaning that you do not second-guess yourself and just write any words and phrases you think of that are or are not associated with your topic. Only after you have exhausted yourself do you go through what you have written and cut ideas that won't work.
Freewriting, unlike brainstorming, involves writing complete sentences. In this case, you write whatever comes to mind to allow yourself to process your various thoughts on the topic as a very rough original draft of your essay. Afterward, you can cut content that does not work and possibly keep some of what you've written for the next steps of your essay.
With an idea web, you write your main topic inside a circle in the center of a piece of paper and then draw lines to new circles with new, related ideas inside them. The plan here is that with each step away from the center, the ideas are getting more and more specific. This is a helpful method for organizing your thoughts from the beginning, as it functions similarly to an outline.
No matter what method of prewriting you choose, you want to encourage creativity and avoid limiting yourself as you write. You never know what interesting insights you might find if you just let your ideas flow!
For more ideas and information about prewriting, please see the following link: