Decide for every sectioning root element (e.g., body) and every sectioning content element (e.g., article) if it contains content which is appropriate for header. The spec defines that it’s for "introductory content", and gives the following examples:
introductory or navigational aids
heading (an h1–h6 element) (not required)
table of contents, a search form, or any relevant logos
If there is such content, use a header element. If the content in that sectioning root/content element is scattered, use several header elements.
Keep in mind that the header always applies for "its nearest ancestor sectioning content or sectioning root element":