Gather notes into one searchable workspace, then label each item by topic and source. Do not organize by date alone: two ideas recorded months apart may answer the same reader question. A short title and one-sentence summary for each note make patterns easier to see.
Create broad sections, place related notes beneath them and mark gaps where more research or an interview is needed. The first outline can be imperfect. Its purpose is to help you test a sequence and decide what belongs in the book before committing to polished chapters.