One of the most touted features of iOS 10 is the dramatically improved messages app. With the new messages app Apple ahas dded the Message framework to the iOS SDK enabling the creation of custom extensions.
Simon Ng at AppCoda has created a step by step guide on how to create a custom sticker app extension for the messages app. You will need Xcode 8 to go through the guide.
You can find the tutorial over on the Appcoda blog.
A nice introduction to creating sticker pack extensions for the new Messages app.