Leonard Cassuto offers reasonably good advice on preparing for, and preparing, a dissertation proposal. Standards and expectations vary from field to field and department to department (even within a field) so take the advice with some caution, but as he says, “consult your advisor”!
In a commentary on Cassuto’s article, Daren Brabham adds this very important point:
I want to emphasize that the prospectus document is meant to get you to the meeting so you can talk about your project in front of a panel of experts. The meeting is meant to help clarify issues that may have occluded your view and to engage in a conversation with your committee about the work you plan on doing. Defense, may in fact be the wrong posture for these meetings. I found that my committee members asked tough and important questions, listened carefully to my responses, and pushed me–all of this was not to make me defensive about my project but to aid in widening my field of vision so that I could see important issues I was missing. What emerged is a set of lingering questions that I must attend to in my dissertation, but the tone of the meeting was never defensive. Instead, I found my meeting to be a rigorous and challenging conversation with experts in the field. This conversation model is important because as we progress beyond comps and through the dissertation process we emerge as colleagues instead of students. These meetings, as conversations, help facilitate that movement.