How do I quote multiple posts?

To reply to multiple posts at once, click on the Plus symbol (highlighted in the image below) along with the plus buttons associated with each of the posts you wish to quote.

Once all the posts are selected, click on the little window that appears in the lower right of the page that shows the number of posts you have quoted (see image below with red box highlighting the button).

 Each quoted post will then appear in their own blue boxes within the editor box at the bottom of the page. Then see this article on "How do I quote a single post?" to edit the quoted text of "Submit Reply".