If you hit the quote button(lower right corner of each post) in the post you want to quote it will open a reply field.
Click below the quote that you see and enter your response.
If you want to quote more than 1 person click the Plus button(next to the word quote) in all the quotes you want except the last. For the last quote use the "Quote" button. That will transfer all the quotes to a reply field.
Hope that made sense.