Using stickers in iMessage is awesome fun, but the functionality is not always that straight forwards.
This video from MacRumors introduces stickers and how to use them.
1: Press + Button
2: Choose the shape of your sticker
3: Press Button To Take Sticker Photo
4: Press Right Arrow To Create Sticker
5: Your New Sticker Will Be Available
6: Long Press On A Sticker To Pop It Off
7: Place Your Sticker On A Message
8: That's It!