TL;DR - No programming needed.
- Make your sticker images.
- Open Xcode, choose the Sticker Pack template project.
- Drag and drop your images into the default Sticker Pack asset folder.
- Name your images and give them accessibility descriptions.
- Choose whether you want the layout to use a small, medium, or large grid presentation.1
This doesn’t seem to affect the actual size the stickers appear to be in the chat area, nor in the grid, but only whether the grid should be spaced for small, medium, or large stickers (4,3,2 across on my iPhone 6s Plus e.g.). This might change in future betas. ↩
Okay, you’ll also need to make some new app icons. There’s about a dozen new app icon sizes needed for Messages apps. But that’s just more design work, not programming. ↩