At the bottom of the comments you want to quote, find the "Reply with quote" button.
To the right is a graphic of a speech bubble with a + next to it. For each comment you want to quote, click the speech bubble.
When you get to the last comment you want to quote, click "Reply with quote." All the comments you selected will appear in the Quick Reply editor at the bottom of the page.
You can compose your reply here, or click "Go Advanced" to edit with more options.