quick overview of how to for Stickers!
1) Open Messages on your iPhone, iPad or iPod touch and tap a conversation in the list or create a new one.
2) Tap the Apps button (it resembles an App Store icon) to the left of the text entry field to reveal the Messages app drawer at the bottom of the interface.
3) You can now:
Select a previously used sticker in Recents
Choose a specific sticker from one of your installed sticker packs
The overview from apple!