To use an app with Messages:
- Open Messages, create a new dialog or tap on an existing conversation.
- Tap the App Store icon alongside the message box in iMessage (or Tap > , then tap App Store Icon)
- Tap icon on the lower left corner and choose the sticker app.
- Tap the app to open it. If you want to add a sticker, touch and hold a sticker, then drag it to the message bubble. The sticker will automatically send once they're added to the message. If you want to use an app in your message, open it or tap the content you want to add to your message. Then tap ↑ to send.
Got a message with a sticker that you don't have? Just tap the name below the sticker to download it.