You need to determine ahead of time what pattern you will use for your headings. For example, are you going to number them—1, 2, 3, 4? Or are you going to use letters? In either case, you will need to decide what pattern you will follow for your subheadings. Some examples are centering major headings and left-blocking sub-heads. But what about if it goes to three or more levels? You must decide what will work for your paper. Remember that what you are trying to achieve is readability and an attractive appearance. In some cases, these matters will be determined for you by the person who assigned the paper or the entity that will publish the writing. What is important is that you have a plan and that you be consistent. Do not do it one way in one part of the paper and another way in another part.