When in text editing mode, simply click-and-drag your mouse cursor to select important keywords, then click on the Highlight button to apply a highlight color to the selected words.
Want to highlight more than one chunk of words? You can do that! Jump back into text editing mode, highlight some more text, and click Highlight once again. To remove it, select the text you'd like to remove the highlight from, and click the clear highlight button.
Want to change the color of your highlight? You can! Just click in to the highlight color editor to select a new color from your palette.