MacStories has a full writeup / tutorial on how to design and launch your own iMessage sticker pack.

I don't know how to code, but I do have some design skills, and I wanted to see what it was like to create something for iOS and launch it on the App Store. So a few months ago I decided that I would make some sticker packs for iOS 10 – and that's what I did. I brainstormed possible ideas, started designing some stickers and ultimately ended up publishing Birthday Celebration Stickers and an app bundle which included World Flag Stickers and a few other country-specific flag stickers.

 The process of making and publishing these sticker packs was fairly straightforward, but I also encountered some unexpected hurdles. To help others who are excited about making their own sticker packs, I've written this guide, which I hope can make the process a little smoother.