I think you should work on this document in the same way as a conference proposal. It is just an abstract, i.e., a statement of what your chapter will be about, why it is important for the scope of the book, and what are the contents you are willing to develop. After that, just work on an academic chapter title proposal which somehow encompasses all your previous thoughts in the abstract.
Congratulations Efrem for this brilliant opportunity which I would like you to grab with glee at the earliest opportunity. I understand you are asking for how you can go a head to submit your book chapter proposal..
All you need to do is to submit the article or manuscript that have written or that you write for the proposed book chapter. The article you write for submission is in fact your proposal for the chapter.
If you the publisher has invited you to do so, simply look at their format or guidelines for the authors. I am sure the editor of the book has already given you the guideline.
If you need additional help on the subject, you can do so. Remember that a book chapter should sections or subsections which include these essentials:
1. Title
2. Name of the author, contact details, and affiliation informations
3. An abstract follow with usually 100-150 words.
4.Introduction which is almost a summary of the work including objectives or the purpose. It contains citations.
5. Methodology or materials and methods
6.Results and Discussion or Results followed by Discussion
7. Conclusions and Recommendations or conclusions followed by recommendations
8. Acknowledgements especially for very key support like finance or funding, if any.
To me a book chapter proposal is analogous to the "Concept note" of a research proposal. You explain what points you are going to respond to in the full length chapter. For example in introduction part of the proposal you specify what you are going to include when you write the chapter.
I think you should work on this document in the same way as a conference proposal. It is just an abstract, i.e., a statement of what your chapter will be about, why it is important for the scope of the book, and what are the contents you are willing to develop. After that, just work on an academic chapter title proposal which somehow encompasses all your previous thoughts in the abstract.