To use InstantVR you need an environment first. The minimum is a flat terrain with a directional light, but you can make it as complex as you want with lots of meshes, rigidbodies and colliders. You are only bound by the limits of Unity and the computer used to drive the game.
You should not include a camera or other first or third person objects in your game. This is fully handles by InstantVR.
Then you should include the InstantVR package which you can download from this site.
To complete the scene you should drag one of the prefabs from the folder InstantVR into your scene.
Now you can press play and walk around your wonderful environment.