There are a couple of ways that you can add to an existing thread. If you want to jump straight in, just use the reply box at the bottom of the page. The “More Options” button also gives you full access to the complete wysiwyg (what you see is what you get) input box with the option to preview your post before submitting - handy to check whether format and links are included correctly.
You can also quote another member’s post if you want to reply directly to something they have said. To do this, click the Speechmarks button under the post that you want to quote and then add your own text below it in the box that appears. While it's often a good idea to cut a quoted post down for brevity, we don't advise editing anything that someone else has said as misquoting is seen as impolite and can cause confusion in the thread.