The goal here is to write continuously for the entire period of time. What you write doesn't have to be on the subject. Writers usually use freewriting to get ready for the process described below. All those things may lead you to your main idea. Once you have a general idea of your topic, you can make a list of everything you want to say about the subject.
Listing is done without concern for spelling, punctuation, or grammar.
After the list is finished to your satisfaction, you can number the points on the list in the order that you want to present them. This numbered list serves the purpose of an outline. Many writers write their first drafts from their numbered list. In order to free your mind to develop ideas, you should not worry about mechanics in the writing of your first draft, unless mechanical errors distract you and you feel compelled to correct them. A collection of correct sentences that have no content sometimes gets you a good grade in high school but almost never does in college.