Now that you have the outline, table of contents or back cover copy. Start taking notes, cut statements from magizines etc. and put them into which ever organization system you choose to create.
Take advantage of moments of stillness, waiting rooms, restaurants when you are alone, business trips etc to address the subject matter.
Strike up conversations with people and do a Q&A. Ask questions that pertain to your chapter headlines, keep writing down all the questions that come to mind about your subject.
Always be ‘working’ on your book and every note, clipping, recording etc. will end up in your research piles or envelop waiting for the next step.