Click the blue text "Reply to [username]" that appears at the bottom-right of the post you wish to reply to.
A new box will appear showing the posted message in quotes.
Now add your reply underneath that (in the same box).
When finished, click the "Answer" button.
However, this is only the case for the desktop website. The "Reply to <username>" button does not appear on the mobile site. In order to achieve the same thing with the mobile site, you have to type (without the spaces in between the square brackets):
[ quotemsg="<username>"]<quoted text>[ /quotemsg] <your reply>