Get your ideas written out on paper in any way that makes sense to you. A written brainstorm might have a main idea circled in a bubble with lightning flashes arching out to ideas that support this idea. You might have a more organized approach, quickly listing a main idea on one line of your paper and listing a supporting idea on each subsequent line. Purdue's Online Writing Lab recommends considering who your audience is and why you're writing the paper before settling on a main thesis.
Organize your ideas using American Psychological Association (APA) style and instructor-recommended hierarchy. According to the APA Manual (6th edition), there are two acceptable formats for an outline. The first uses Roman numerals to identify each heading, capital letters for the sub-headings and Arabic numerals for the sub-subheadings. The other format uses a decimal system in which the main headings are labeled with the chapter number (i.e., 1.0) and each subheading gets a number after the decimal (i.e., 1.1, 1.2). Sub-subheadings are labeled with an additional decimal (i.e., 1.1.1).
Determine whether you are expected to use a full sentence or a heading outline. Heading outlines permit you to simply use your heading for each level of your outline. This makes it easier to track your ideas, ensure they are placed in logical sequence and are strongly supported. As you look at your list, make sure each heading shares similar structure -- if one heading starts with a noun, they should all start with a noun. The APA Manual refers to this as parallelism. Sentence outlines require each level of your outline to contain a fully-developed sentence. This can make bringing your paper together easier as the sentences begin to introduce a flow to the work.
Pay attention to formatting. APA style is strict on formatting, so be sure you have properly double-spaced lines, indented subheadings and double-indented sub-subheadings. In sentence outlines, all sentences should start at the same level as the beginning word of that sentence. In other words, a three-line sentence placed at the indented subhead level should have all three lines starting a tab space in from the left margin. Quotes should be cited and have their associated reference information included at the end of the outline.