But as global overview I want to say globally how to add Steam to your game on Unity.
1. Add to your working plan Steam support. cuz in the future of your game architecture you will need some events from the world to react with steam. I mean be ready to sent events from any part of you code to some abstract Steam object
2. Then pass through greenlight and receive AppID
3. Download Ludosity steamworks wrapped from Assets Store for Free.
4. Setup project according to Ludosity manual.
5. Don't forget to launch Steam client.
6. Upload just one ( even empty project to Steam )
7. Do everything else from the editor.
8. For middle like Anoxemia and small games you will not need to setup local steam client server, simply leave it.
9. Setup everything on steam partners site like achievements, stats, etc.
NOTE: after every publishing changes in steam partners RESTART steam client.
10. Do everything from editor.
Keep in mind that cloud saves are not always working properly from Unity editor. To test normally learn about steam file synchronization, then clear local cache and test on a complete build.
To make everything working on a MacOS and Linux watch this video: