How do I quote a single post?

Simply click on the word Quote below the post you wish to quote (highligted in red in the screen shot below), do not click the Plus sign to its left. That post will then appear in a light blue box within the editor box at the bottom of the page.   

Once you click the"Quote" button you will see a view like this below and an area below the quoted text to write your response, followed by the "Submit Reply" button when you're ready to post:

  If you only want to quote part of a post, you can click inside the blue box and remove the unwanted portion by either selecting it with a mouse (shown highlighted in light blue below) and deleting or using the delete key. Then type your response below the quoted text you've left in.