Just do it, practice and select the back arrow if it stuffs up

When you select the middle icon (next to the word quote one) you will see the plus change to a minus. You then go to each post you want to quote and do the same thing so there are all minus signs below the talking marks of the middle icon (bottom right corner of the post). When you are then ready to post your reply, you need to select the quote button which will take you to the reply screen. I learned by stuffing around and watching what it did.