To add a comment, highlight text in the document. You'll see a new conversation window appear on the right where you can enter your comment. Then press enter/return to save the comment; it will be visible to your classmates immediately.
To edit a comment, click on the comment to edit it. Change the text of your comment as needed, and then press Enter to save your changes.
To delete a comment, click the trash can icon beneath the comment bubble. To avoid disrupting the flow of an ongoing conversation, you can only delete a comment until someone has responded to it.