How to use iMessage Stickers:
• Tap the App Store icon alongside the message box in iMessage to open the app drawer.
• Swipe left or right over the apps to find the one that you want to use. Then 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 you add it to the message.
If you want to send a sticker in your message, open it or tap the sticker that you want to add to your message.
• Tap the upwards arrow to send.