Quoting a Reply is a good way to give context to your reply. Especially if you are responding to a post that hasn't been posted recently.
Note: You cannot quote the first post on a thread.
Step 1: Find a Post you want to quote
You can quote any reply to a thread, so just find a reply you want to quote.
Step 2: Click the quote button
This is located below the main body of text of the message and right of the reply button. Click this button.
Step 3: Type your reply
From here it acts just like a normal reply, so just type your reply and hit the reply button.