Index after, not before
An empty notebook can invite an elaborate filing system. Resist planning categories before you know what the book will contain. Number the pages as you use them and leave two pages at the back for a simple index. A few recurring subjects will usually emerge on their own.
Use ordinary words you would think to search for later: garden, repairs, reading, routes. A clever abbreviation saves little if you cannot remember it six months from now. Combine neighboring subjects only when they genuinely belong together.
Point to a useful page
An index entry can be as short as "bicycle light, 18, 43." If a page contains only a passing mention, it may not deserve an entry. The aim is not an exhaustive record of every noun; it is a set of reliable routes to the things you are likely to revisit.
For a subject spread across several pages, add a tiny forward reference at the end of each entry. "Continued on 43" works without a special symbol. Leave the original context intact rather than copying everything into a more orderly but less informative summary.
Close the book gently
Write the date range on the inside cover when the book is full. Add two or three words that distinguish it from its neighbors. Store it somewhere dry and away from direct sunlight. A finished notebook does not need a ceremonial archive; it needs to remain readable and findable.
Before starting the next book, transfer only unfinished tasks and reference information you need at hand. Let completed work stay completed. The new first page should feel like room to think, not a backlog imported from the old one.