simple animation for repeated object

Hello,
I need to learn how to add a simple loop animation with a prefab. Example: a 3d "go to" arrow that bobs up and down. Once learnt I will use the same method again and again for all kinds of similar prefabs. There must be a tutorial or a post that has a step by step. I was disappointed in the Unity 3D tutorial on this topic - it uses a pre made animation - does not show how to start from scratch (the sliding door in stealth).

I am NOT referring to legacy. I would like to use the new correct method - I guess it would be an idle state. But that "idle state" is constantly applied no matter what happens to the object. Then again, I probably do not understand states?!

Please help. If a good example exists showing the above already then we can delete this post - after I get the URL ... : )