Welcome, Patrick.
First of all, let me start by saying I'm by no means an expert, so this is the R&D of someone in the throes of writing a non-fiction How To. Take away from it what you will. But as your first response, perhaps it will get the ball rolling.
Google has been my close buddy, my writing partner through this in both research (on a subject with which I am very familiar, but need to fact check all the time), finding outline/proposal/Table of Contents help and in researching other books on the subject. Of which, I'm happy to say, there really aren't any, but in my field there were several books that guided me in my own process.
If you Google non-fiction book outline you'll find a number of informational sites. Basically, the outline should be an expansion of your chapters. You have the Table of Contents, which is your section headings or chapters. Under each chapter, you should expand this to include the sub-headings or areas (in, e.g. slightly expanded bullet point) you will cover in the chapter. This helps organize your own thoughts and direction, and tells your potential agent/publisher more about the subject and whether you have the chops to write it.
I also used Amazon to find books in my generally related field and others that weren't, but looked interesting, to search within the book for their Table of Contents for layout and ideas, or sometimes typed in key words to see excerpts and how it aligned with what they'd written in Table of Contents or how they laid it out. If you have ever bought anything from Amazon they'll let you look at more of the book than if you haven't, but you still have the option of researching published books in your field and at least seeing the Table of Contents.
The other important proposal factors are writing samples (either previously published or at least the first chapter of this book) and your bio.
I hope this is helpful. Best of luck!
