A proposal describes your project from both inside and outside. First, the inside stuff:
A proposal puts forth your argument. It points toward how it will be proved, giving well-chosen examples without unspooling them in detail. A few exemplary details will help illustrate your presentation, but a profusion of them will distract. Such details serve the purpose of demonstrating—not fully proving—your argument.
A proposal describes how your argument will fit together. What examples will you use, in what order, and why? How is the argument sequenced and subordinated? You will probably need to provide a chapter outline, but you should offer a clear and extended overview of your argument long before that.
A proposal outlines methodology. How will you make your argument? What theoretical, historical, contextual, and interpretative tools will you use? Will you employ any particular approach?
Your proposal should fit your dissertation topic. A proposal to edit a scholarly edition, to pick one exceptional possibility, will require a different presentation than a dissertation laid out in the model of a monograph (introduction plus four chapters on related topics). The shoe must fit the foot and not the other way around.
From the outside:
You need to show the place of your dissertation in the critical field. Which field and subfield conversations will your project enter, and how? Which critics will you be building on, and which ones will you be revising? Your dissertation marks your formal entry into the community of scholars, a world of intellectuals engaging in overlapping conversations of varying size and scope. Your proposal must show your awareness of those multiple discourses and show the place your research will occupy within them.
Accordingly, you should include a thorough bibliography in your proposal so that readers may look at what works you plan to consult, as well as those you have consulted already. Your committee will review that list and use it as the basis for further suggestions.
Finally, I offer proposal writers a commandment and a postulate.
The commandment: Consult your adviser as you develop your proposal. The myth of the writer as solitary genius striving away in the garret has surprising persistence. I’ve seen many graduate students teach their undergraduates to collaborate without realizing that they’re not following their own advice. (That is a mistake I made often enough myself.) You should not imagine that you will be writing your proposal on your own. Instead, draw on the experience of your peers, and especially your adviser, as you shape your topic so that it may be the most relevant, the most challenging, and the most marketable later on.
The postulate: Your dissertation will be different from your proposal. That’s to be expected—and the differences can be substantial. Your proposal outlines a hypothetical dissertation: what your thesis looks like to you from where you stand now. The goal of a proposal is not that it should outline your future dissertation. Rather, it should outline one possible dissertation, and do so plausibly.
If you can offer up a credible possible dissertation based on your ideas, then it follows that the dissertation you actually wind up writing will benefit from this early exercise. Your proposal will get finished faster, and so will your dissertation—because unlike diamonds, dissertation proposals (and dissertations) are not forever. And graduate school shouldn’t be, either.