Use your mouse or trackpad to select the section of text you want to highlight. As you can see in the image below, when you select text, a pop-up appears that says "Highlight." Click the color you want, and your selected text will be highlighted.

To remove a highlight, click the highlighted area, then click remove.

There is also a "bubble" icon to the right of the text—when you hover over a text area—that will allow you to attach a comment to that paragraph.

When you click on the "bubble" icon, you will see the following pop-up: