This is a very simple example of how to setup Facepunch.Steamworks in Unity3D
First of all, you'd need to import the Facepunch package (you can head to Releases, and just get the UnityPackage file)
Next you'll need to copy the native dll files to the root of your UnityProject (where the Assets/Library/etc folders are)
After that you should be able to just attach the SteamClient script to any object in a scene, and fill in an AppID. The SteamClient script is the one provided by Facepunch, I did not make this.
Tested in Unity 5.6 and 2017.1