Documentation sounds like a dirty word. It can take time for you and for the professionals reading it if you make it too complicated. I have created a system that doesn't take as much time for me or for the professionals. It gets the information to the professionals in a way that gives them an accurate picture of your child. Medication and other recommendations are informed decisions instead of a gamble. These methods shared in my upcoming book.
Either in a notebook of some kind or on your computer, start making brief entries of your child's day. Include things like behaviors, sleep, problems at home or at school and medication changes. Keep it simple. The journal is for your eyes only so it doesn't need to be fancy or time consuming. You can use it later to create the document to give to professionals.