You will need to be logged in to the forum. When viewing the post you wish to reply to, scroll to the bottom and simply enter your text in the message body at the bottom of the page and then click on the Post reply button near the bottom right of the screen.
NOTE: It is also possible to include a previous post as a quote in your reply. To do this, click on the Reply button at the bottom right-hand side of the post you wish to quote.
NOTE: It is also possible to include a previous post as a quote in your reply. To do this, click on the Reply button at the bottom right-hand side of the post you wish to quote.