Step 1. Explore and understand competing titles.
Step 2. Research the non-book landscape.
Step 3. Study the authors and influencers you’ve found.
Step 4. Pinpoint your primary market.
Step 5. Analyze how you reach readers.

For the full article, including details about how to take the steps, downloadable worksheets and lots of other useful advice, see the ever-informative http://janefriedman.com/2014/07/09/5-research-steps-write-book-proposal/