Click "Reply to this topic" which is a black box at the top of the thread, it'll take you to a reply field. Type your reply then click "Submit reply"
Scroll to the bottom and write your reply in the speech bubble next to your profile picture, then click "Submit reply"
If you want to reply to a particular WHQer, you can tag them in your reply by typing @ plus their username. For example, to tag me you'd type @PoppyWHQ and I'd get a notification to tell me about your reply.
If you want to quote another WHQer in your reply, highlight the text you want to quote and you should see a "Quote selection" bubble pop up. Click "Quote selection" and it will automatically paste the quote into your reply. The WHQer you quoted will get a notification about your reply.