First, find whichever post it is that you wish to quote.
On the lower right hand side of that post, you'll notice a button marked "View Post." Click on it, and the post will appear on its own separate page
Now just copy the url, paste it into whichever thread you wish to post it in, and you'll have a link directly to that post. You may also choose to copy and paste the relevant sections of the quoted post into your own, as a courtesy to other users, although that is up to you.