Q: HOW TO QUOTE

How do you take say, part of a post within a thread (say Post #6 take part of the text/pic and copy it in a post further down in say #20) and then comment on it.
I've seen it in other threads where the text is in a box and the comment is below the box.
Can't seem to be able to do it. Tried highlighting text in blue, hit quote, went down to new post, hit insert quote, multiple items appeared, but not the right one.
thanks
tom