The Highlighter tool is very easy to use in GoodNotes. Select it and draw over any words or objects on the page to highlight them.
Changing colors and thickness
The Pen, the Highlighter, and the Shape tools each have their own set of shortcuts for colors and thicknesses in the contextual menu (the right-hand side section).
Customize a color or thickness slot by tapping it twice.
Draw in Straight Line
Tap the Highlighter again while it's selected to access this setting. If this is activated, all lines you draw with the highlighter will be straightened automatically.