Probably the easiest thing to do would be to go to the test area on this site and experiment. I would suggest that you create a new message in the test area and use the "quote" button above. This opens up a script prompt window where you can enter in a line of text.
This line was entered in the script prompt.....
The above line was entered into this message in exactly that fashion. I clicked on the "quote button above, entered "This line was entered in the script prompt....." in the line provided, then clicked the "OK" button. This then plugged that line onto the bottom of the message.
Now, since you are in the TEST area, you are allowed to edit your own message. Click on the "edit" button in the lower right hand part of the screen when you are viewing your message. Ignore the "quote" button that is there for the moment.
When you are in edit mode in your message, take a look at what was done with that text line entered using the "quote" button. See that it is surrounded by command tags that tell the system to display that line as a "quote"? You can bypass using the button and manually enter in those opening and closing tags yourself. But try not to make a mistake, because it can really garble up your message if you mess it up. But this is an easy way to cut and paste sections of messages into a reply once you get used to doing it.
Now, back to the "quote" button in the lower right hand corner of the screen while you are viewing a message. Clicking on THAT particular button will capture everything entered into the message, and begin a new reply to the thread with the captured text marked in the quote brackets. In a long thread, this will work on ANY message within the thread, allowing you to start a reply referencing a particular message by simply copying it into quotes and beginning your reply message.
Does that help? You may have to play around with it a bit, but I'm sure you'll get the hang of it pretty quickly once you play with it for a while.
Good luck!