You haven't waited too long to start your book. You can start anytime by just entering a history of what you have done to date. Just a history of what you have done that shows you have progressed to this date. Fill in the dates as near as you can remember. Be sure and make a few sketches that show any changes, or improvements, if that is true.
Photos showing prototype improvements are a good idea.
I know I'm asking a lot, but you need good documentation or you might wish you had if you're ever in court. Then it will be too late. It's simply developing good habits. Then every time you make another entry have it witnessed again, on the same page that you sign. Fill in blank areas with diagonal lines so that there are no spaces where one can say you made the entry after it was signed.