I've been doing this for years.
First couple of pages - each page 3 month overlook for 18 months. This is where every single appointment I have goes, including work stuff, school holidays, parents meeting, etc. Then a gift idea list.
The month is set up as a weekly overview, which I quickly add every week as my weeks vary space-wise. Some weeks I can do a single page, other weeks I need a couple of them.
I do have a monthly tracker - meds, fluids in/out, sleep and a few other things I have to keep an eye on.
A list which is colour coded for priority.
Then whatever for that month - finances, if I've seen/tried a new recipe, books and films, specific shopping items, a brief diary recap if it relates to something I'm tracking, including the dc's. Design ideas for anything I might want to do in the future, appointment notes and much more.
This time of the year, every single thing needed is written down from the carrots to the wrapping paper. Last delivery days for companies I use. If people are coming here, who they are, what they are bringing, allergies etc.
Expensive and electrical purchases with all relevant details as also added, simply for if things go wrong, whilst the original paperwork is being dugout, I'm already on hold. These get carried over to any new book at the beginning until a reasonable time has run out. If I still bought warranties I would do the same to keep track.
I do index everything and do this every time I start a new page. Makes finding things months later a lot easier. If I'm going into a meeting and know I need previous info, I mark the pages with some washie tape, and the index helps in the meeting if I need to refer to something unmarked. If I have referred to something in my private diary, I will mark the page and somewhere the page and book number is mentioned.
I've used a variety of paper and sizes. Prefer A5 simply because it fits in my bag. Paper, thick enough so nothing seeps through the page. I tried the filofax as finding books cheaply with 500+ pages can be challenging, but found it harder to flick through quickly in the same way you can a book.
I've also tried doing it digitally, it does have it's advantages as you can search, or click the index part, but it's not the same. Although I do keep playing around as my skills develop and more note/diary options are released.
So my advice would be - start with the yearly overlook. Add in the monthly or weekly diary for ease, and just start each page blank. It soon becomes obvious how you want yours to work. That's what I love about the system, it adapts to you because it's all blank aside from maybe a page number, a dot or a line.