Posting a reply to a thread is pretty straightforward. Select the thread you want to reply to. Scroll to the very bottom of the page. You should then see a "Quick Reply" form. It's circled in the image below:



Once you've entered your content, click "Post Reply." Your reply should instantly be added to the thread.

Alternatively, you can click the "More Options" link to use a more detailed post editor. You can also then preview your post before submission.