In terms of making the book itself, it depends on what kind of book you're after, anything can be used as a book of shadows, something as simple as a notebook, to something as complex as a handmade leather bound book.
In terms of the content, it's all about you and your personal practice, what you put in it is solely up to you, though generally they contain the notes and findings of each persons magickal practice.
Don't forget that a BoS can also be digital, either on a Word document, google docs, etc. This method of BoS-keeping is less traditional but very ideal for broom closet dwellers. Some people choose to have digital backups of their physical one.
Personally I have my BoS in the form of a Word document, but I've transitioned onto www.wattpad.com as I find it much easier to edit and swap things around. I also don't have to worry about backing it up onto clouds or hard drives since Wattpad backs up everything for you :)
In terms of content, mine is split into parts:
> Fundamentals (visualisation, grounding, centring, etc)
> Religion (Wicca, Norse Paganism, Helenism, etc)
> Tools and Correspondences (colours, crystals, altar tools, etc)
> Divination (tarot, runes, numerology, etc)
> Spellwork (how to cast spells, types of spells, etc)
Every BoS is different, so you can arrange it however you like. Hope this helped :)
I prefer to use notebooks to make my BOS, for various reasons most notably the fact that I believe that when you write something yourself, it has a sort of power to it. Seeing my own handwriting in a book that I spend much of my time adding on to and coming back to has a sort of special feeling to me.
My own BOS is a black leather notebook that I found at a bookstore. I'm about halfway through filling up the pages, and so far it's looking great.
Of course, this isn't the only way. I'm just sharing my personal views. I do believe that making a BOS online or in a binder may be convenient or faster in a lot of cases, especially when I want to move a section of my writing to another part of the book.
I usually buy the really ornate (and rather cheap) notebooks from Peter Pauper Press and use those. When I was younger and still in the broom closet I made my own alphabet and wrote in that exclusively when writing anything to myself, be it as mundane as a to-do list or the first pages of my first book of shadows until it was just as easy as writing in the alphabet we're all reading right now.
You can put whatever you want in a book of shadows, like how I absolutely refuse to put correspondences in mine. Instead I alternate between poetry or Psalms and articles concerning how to do something like sigils or divination.
Making an addition on the reply by Ashen_Heart, A BoS can also be on something like a OneNote notebook or a Google slides slideshow. Mine's on a OneNote notebook.
