Write out a brief set of issues you expect your thesis to address, then narrow them down until you are left with a reasonably narrow topic. A doctoral thesis may be a lengthy document, but it is intended to be a comprehensive examination of a fairly narrow topic. One common early mistake among thesis writers is to attempt to address too many issues at once, thus losing focus and losing the ability to write a concise work of scholarship. If your topic is too vague or broad, your thesis will lack direction when you begin your draft. For example, if you are addressing the history of transportation systems, pick a specific period of time, location, type of transportation, or a more specific aspect of transportation. Go through this process until you are left with a subject that can be addressed within a few hundred pages at most.
Make a brief, bullet-point sketch of specific issues and topics within the narrowed subject area. You may find that the number of issues is still too wide and you need to trim down your subject area again. Or you may find the opposite and will want to broaden your subject a bit. As you sketch out more and more issues, you may organize them in any manner that makes logical sense, such as chronology or complexity or category. If you find it difficult to develop this sketch, you may want to proceed to the next step and then return to your sketch again.
Briefly review the academic literature related to your subject and the issues you have identified within your subject. This will not be a formal literature review, which is a separate and much lengthier process that may also be required as a part of your thesis. Instead, conduct a quick search of the academic databases and libraries available to you using keywords related to your topic. This is important in order to make sure that you are not directly duplicating an existing academic work, and to ensure that there are enough resources for you to write your thesis in a timely fashion. If there are few or no resources, your thesis may end up requiring a large quantity of original research, which may be beyond your desires or capabilities.
Expand and organize your initial sketch of your topic, using the results of your literature search to help devise specific topic headings. After each topic heading, write a short paragraph describing the information you expect or would like to present in your thesis relating to that topic. Use formal Roman numeral section and subsection headings, and add more subsections as necessary beneath each topic. As you go through each issue, your sketch will become a preliminary synopsis of your thesis.
Polish your rough draft of an outline into a formal synopsis presentable to your academic advisors. By now, you should have an outline containing an overview of the subject that will be addressed in your thesis; add a title page, an index, an abstract of your thesis proposal, an introduction to your subject, your proposed methodologies, the topic headings and brief paragraphs contained in your outline, a summary of your preliminary literature search, any proposed conclusions you can present at that stage, and your references. Include a brief discussion of relevant theories that you intend to address in your thesis.