Sending a reply is simple! Just open the unlocked message you wish to reply to and press the reply arrow in the bottom right of the screen. The coin figure by the arrow shows how many coins it costs to reply to the user.
Once you press the reply button, you will be taken to the camera. Just take your picture or video, swipe to add filters or tap to add text, and press next.
You will also have the option to add a tip, just move the coin along the wheel to select how many further coins you wish to send!