A picture tells more than thousand words
(iPad only)

You create a new sketch by selecting this option from a book view’s + menu. This will give you a white canvas, a selection of twelve colors and a brush of variable thickness. You can start drawing right away.

If you want to change the color or thickness of the brush, tap the brush icon in the toolbar and change the settings in the popup window. You also find buttons for Undo and Redo in this window. When you have finished drawing, tap on the OK button in the top right corner.

To continue drawing, tap on the little pencil icon in the info bar.

To modify an existing photo, image or sketch in Notebooks, tap the small pencil icon in the info bar and comment photos or mark important spots.
Changes are saved to the original photo, so you might consider creating a copy first. You can use Notebooks Revert to Previous action to undo all changes.