Press and hold text on screen, then drag the sliders on each side of the text to select the amount of text that you want. Then, in the menu athat has appeared, select the colour of the highlight that you want to add. The text will now be highlighted, and a tappable target will be added. In the dialogue that appears you can add a title or notes for this highlight. You can also choose whether you want this text to always be highlighted, or only when you tap on its target.