Like this....
"reply with quote" at the bottom of their post, then just delete the parts you don't want to "quote"
Or, you can highlight the part you want, do a "copy", then paste it in between the words "quote" in brackets that you get when you hit the "wrap quote tags around selected text" button at the far right of your toolbar at the top of your reply box.
was that complex enough?
The first method is easiest. It's considered "good form" to put some kind of comment in the quote, though.... saying something like "edited for length", just so others will know that it is not a complete quote... helps to keep anyone from accusing you of "mis-quoting" someone..