I have been inddexing my paper notebooks for years. Here is a systematic description of how someone else does it.
I do like the idea to assign every notebook not just a year, but also a letter: "I letter the volumes in a year because it helps me locate a volume quickly on the shelf: N7 is volume N from 2007. Before indexing digitally a date span was all I put on a journal cover. There are many other approaches that would work."
I am not sure that you should not go back and index old journals. "What is past is past and whatever organizational system you used will have to suffice. If you try to go back and index past journals (even if you are new to journaling and have only a few volumes to index) you'll become bogged down in the cataloging of your past instead of being involved in the observation of your present. That is never a happy exchange." But it need not be an "exchange." The past may enlighten the present and vice versa.
By way of thecramped! Now, that wasn't hard.