How do I write my book of shadows?

Use your book of shadows as a magickal diary - record in there the actions you take, rituals, spells, the results of these, magickal information you learn about. Use it to keep a note of anything you feel is important within your spiritual path. You can separate the different parts into Rituals, Spells, Seasonal Celebrations, Magickal Correspondences (oils, herbs, colours etc for use in specific circumstances), Notes, and so on.