Open ChitChat app.
The message you have received will have a red notification, or it will be on top of the message list.
Click on the message.
You will be taken to the message window.
You can now write your respond and send it.
Bonus tip: Did you know that you can send giphys in ChitChat? To try it, write this character / and then giphy. Like this: /giphy. When the blue icon appears, type in a word to search and when you see one you like, hit send.