Q: How do I develop an outline for a Masters thesis proposal?

Since this is just an outline and since your professor has not specified any format, I think the best you can do is to follow the basic structure of a research proposal. This will help you collect your ideas and give you a sense of direction. 

Generally speaking, there is no standard format for a Masters proposal; each university has its own preferred format. For example, you can check the formats followed by the University of Westminster and the University of Birmingham. You can check your university's website to see if they recommend any specific format. However, since your professor did not mention any, it is possible that your university does not have any preferences in terms of format.

A Masters thesis proposal is somewhat similar to other forms of academic writing; ideally, it should include the following:

• Introduction: The introduction provides background information and sets the context. It also includes the rationale of the research, that is, why you are conducting this study.
• Problem statement/research question: The introduction is followed by the problem statement, briefly explaining the problem or gap in research that your study will address.
• Methods: Next, you should explain the proposed study design and methodology that you plan to use for your research
• Outcome and significance: Finally, you should describe what you think the supposed outcome will be and explain how it will add to the existing knowledge in the field
• Timeframe: Also mention a tentative time frame for the project and how you plan to work within that time frame.