When I made my BOS, I got a recycled earthbound notebook( whichI just thought looked cool), I decorated the outside of it red.
So far I have put things on divination, herbalism,The charges of the God/Goddess, and some other things, and I treat it like a journal.
I put my experiences in it, the spells that work for me, and whenever I can I put my energy into it.
I try and make it a habit to read up on a certain subject every week and put it in there, it helps.
I also put my Patron Goddess, and the day I was dedicated, in there.
Just make sure you purify it and cleanse when you first get it, so that it can be cleansed and "fresh".