I just posted a video about this, so check out my YouTube channel if you want to see it.
My BOS isn't anywhere near complete (and really, it won't be until I die). I have so much stuff I need to add from my Mirror Book, files on my old laptop, etc., but oh well.
My BOS is a 3-ring binder with dividers in it. The cover isn't decorated or anything. The first page is sort of a title page that just says "Wyllow Lylly's Book of Shadows" or something like that. Then I have my dedication/book blessing to Hecate. Then comes my "Beliefs & Beginnings" tab that has my beliefs about life and death, deities, the 13 principles of Wiccan belief (though I'm not Wicca, I like those), the Rede (again, I like it, but I don't necessary totally agree with and follow it), circle casting, cleansing, consecrating, charging, quarter calls (don't really use those now, though), tools and what they're used for, general Pagan beliefs, etc.
Then I just have some meditations. I need to add a few more to that section.
Then comes the section on deities and other important spirits (to me).
Then some stone meanings, herbs, seasonal correspondences, sun signs, moon signs, planets, days of the week, phases of the moon, symbols, Theban Script/The Witches' Alphabet, etc.
Then I have a small section for chants and poetry.
Then and unlabeled tab that just has a ton of information about Air right now.
Then spells, then rituals, then Sabbats, then Esbats, then divination, then recipes and last I have stuff about crafts.