When you quote some thing which I assume you now how to do, it opens up a response section at the bottom of the screen, the top of the screen has the original thread you were in when you quoted. At the top just change to another thread either by searching or going to latest and finding the thread you want. Open the thread you want the quote to reply into.
Write your message under the quote in the bottom section and hit ‘reply’ when you do this a prompt box will appear and ask you which thread you want to reply into. The one you are in now or the original thread you quoted from. Click on the one you want.
Hope this makes sense. Maybe try quoting this into my thread.