can someone teach and show me how i could do this?

It's too late here for me to write any code, but it's a matter of projecting angles onto different coordinate spaces.

You could do it with the transform basis, which tells you the player's coordinate space. If you take the cup object as a vector, you and transform that (project it) onto the players space.

That give you a relative direction (relative to the player) then you can convert that to an angle like 90 degrees. Check the documentation for Transform, Basis, and Vector3.