How to do Multi-quote...

The button for the Multi-quote option is located to the right of the "Reply with Quote" button.

For this function to operate, You have to click the multi-quote button for all the people who have posted in which you want to include in your reply.

Then on the final post (person), you click the multi-quote AND the "Reply with Quote". When the box opens for you to write your reply, ALL the posts you had checked for the multi-quote, will be in the box for you.

Once you reply, all the multi-quote checks will be reset. So if you happen to discard the reply, go back and reclick all the posts you included in the multi-quote to "un-quote" them all. Otherwise, if you quote a single post later, all those previous multi-quoted posts will show up in the reply.