How to use iMessage's new features, effects, and stickers

Apple’s new iOS 10 update adds a bunch of fun new features to iMessage, including effects, apps, and stickers that you can send to your friends. Unfortunately, it's not immediately obvious how any of this actually works. Confused how to add stickers? Want to send messages with confetti? Read on!
iMessage Apps
The biggest change in iMessage is the addition of apps, which fall into roughly two buckets. There are more traditional app integrations, like Fandango or Venmo. These allow access to