Use Roman numerals to list the core ideas of each section of your paper. Put a period after each Roman numeral, such as "III." Write in parallel structure. Each heading should contain only one central topic. The number of headings depends on the scale of your thesis. In general, a thesis should have at least 5 main sections. APA does not emphasize a strict formatting rule for the outline; however, many prefer to capitalize the first letter of each word on the heading.
Indent the line following the heading and list the subheadings using capital letters. Put a period after each capital letter, such as "A." Subheadings contain more specific information about each section. Like the headings, write in parallel structure. You may have more than one subheading under each heading.
If you need to put more details for the subheading, indent the line following the subheading. Use Arab numerals and follow with a period to continue jotting down specific detail of the subheading. Similarly, for more subordination of the sub-sub-heading, use lower case letters followed by Arab numerals in parenthesis. For example, you may have I. A. 1. a. (1)
Some professors require students to write outlines in complete sentences. In terms of the alpha-numeric listing, some professors also require decimal outline, such as 1.0, 1.1, 1.2, etc. Decimal outline is more common in the sciences. Consult your professor for specific outline formatting instructions.