Your Supervisor may have specific requirements about the nature of your prospectus, so make sure to check with them! Barring that, here is a good template you can use:
Cover Page: the Title, your name, and the names of your committee members.
Abstract: 200-500 words summarizing the overall project. *Tip: write this last.
Contribution of the Dissertation: This is the section where you lay out how your project is bringing something new to the field or topic in question. This will obviously require a discussion of the state of the field or the argument, against which you are positioning yourself. You may want to give some thought in advance to what work you will discuss here, so you can avoid overlap with your next section (or at least, use the overlap somehow).
Literature Review: This is the section where you review the literature related to your topic.
Chapter Outline: Probably the funnest and hardest section to write. You may be surprised at how long it takes you to figure out how to organize things into chapters. (This section can also be tricky, as it confronts you with the question of objects.)
Schedule: Here, you'll provide a projected breakdown of your remaining time at Harvard. Beyond figuring out how you'll order the writing of the chapters (and giving yourself dates of completion for each one!), you'll also want to write here of your plan concerning finances. Do you need to travel for your research? When are you going to do that? Are you going to apply for certain grants? Which ones? Etc.
Bibliography: You got this.
The Dissertation Prospectus
After the successful completion of the general examinations, a topic for the dissertation is chosen in consultation with the student’s dissertation director and advisers. A committee is to be formed and agreed on no later than January in the third year of study. Once a student has a topic and an advisor, a formal written dissertation proposal is the next step. Excluding the title page and bibliography, the prospectus should be 1,500-1,800 words (5-6 pages), not more. Students are expected to have their prospectuses approved within five months of passing the general examination in order to maintain satisfactory standing.