Level One headings are used for the largest sections of the paper. Level Two headings are used for the largest subsections of each Level One section. Level Three headings mark subsections of Level Two sections, and Level Four headings mark subsections of Level Three sections, and so on. The amount of levels you include depends on the length and depth of your paper.
The introduction does not receive a heading.
Do not use numbers or letters to indicate sections or identify headings.
Don’t go hog-wild with headings! Use them only when it benefits the overall organization and readability of the paper.