On your iPhone and iPad you can add quick drawings and handwriting notes your documents. You can access the Sketch block by tapping the plus icon on the bottom of the screen - Apple Pencil also supported for iPad.

After clicking the plus button you will see the different type of blocks you can add to your document

There are multiple block types and options you can add here, but you need to select Sketch if you would like to add a drawing.

When sketching you can find pencil, highlighter, and eraser. There are different color options as well.

Important: When you are done, tap on the Done button and select Save Changes.