Your dissertation committee will be in charge of the specific guidelines of your proposal (length, format, expected scope, etc.). That being said, all effective proposals do at least four things: Contextualize and ask an answerable question; identify what existing scholarship has and has not done to address that question; explain how the researcher will answer their research question (as well as why that’s the best way of doing things); and elaborate on the significance of the proposed research.
This worksheet will give you a few questions that you should come back to again and again as you begin to outline your proposal. It’s also a good primer for understanding the general logic of a proposal if you are unfamiliar with the genre.