How do I write a comparison paper proposal?

If you have received an assignment to write a comparison paper, the first thing you should do is read everything your teacher or professor has given you pertaining to the assignment. You may have an assignment handout, an online description of the requirements on your class learning platform, or a section of your syllabus that gives details on how to complete the assignment. In addition, you may have been given a reading assignment in your textbook that will help you.
A comparison paper is a composition that analyzes two subjects for their similarities. If you have been given free rein in choosing your topics, choose something you know a lot about already—either as a personal interest or something you've learned about. If you love literature, you could compare two poems, two short stories, or two novels. If you like art or music, compare two paintings or two songs. If you're a business major, compare two different companies or stores. To make your paper more interesting, choose subjects that seem quite different. Your paper won't be intriguing if you compare two things that obviously have a lot in common.
Be sure that your assignment is to write a comparison paper and not a comparison-contrast paper. In the former, you will only be looking for similarities. In the latter, you'll discuss both similarities and differences. If you are writing a comparison-contrast paper, you may want to discuss all the similarities in one section and the differences in another. Another way to organize such a paper is to take one characteristic of the two subjects and discuss its similarities and differences and then take another characteristic and discuss that one. For example, if you are analyzing two coffee shop chains, you could discuss the similarities and differences of their products, their customer loyalty programs, and their decor.
You also mention a proposal. If you are just working on a proposal for your paper, that is similar to an abstract or précis. You should prepare an outline of the key points of your paper and then write a sentence or two about what you intend to cover in each section. Depending on what your instructor has asked for, this summary of what you plan to write could be just one paragraph or up to a page or two.
If you also have the opportunity to write a proposal paper, that is a completely different type of composition. A proposal is a problem-solution paper. You will find a problem that is fixable and propose a solution. In your paper, you will explain the problem, describe who is affected by it and who has power to improve the situation, and suggest a course of action to solve the problem. Your proposal will include logistics that describe specific actions, resources, and approvals that will be necessary to implement your solution. You will also evaluate at least one alternate solution and explain why your plan is superior.
The most important thing to do before you start writing is to clarify the assignment. Make sure you know whether you're being asked to write a comparison paper, a comparison-contrast paper, or a proposal paper.