Hanging a mobile is very simple. Just keep in mind a few things.

1) Inspect the mobile; all the wires are supposed to be leveled. The strings are glued to the wires, however, it could become detached partially from the wires and shift during shipping. In which case, all you have to do is to move the knots towards the lower point. 

2) Choose a spot. Avoid areas where there are a lot of air movements. For example, areas in front of an air conditioner or a fan, in front of a door and directly above a radiator.

3) Find a simple cuphook.

4) Screw it into where you'd like to hang a mobile. Hang the mobile using the eye of the fish.

5) Adjust the string length according to the ceiling height.

And you are done!