You use a voltage or current source between the object you want to protect and the object you do not care being consumed. Apply the polarity so that the consumed object gives off its atoms to the soil or liquid. This requires knowledge of the electro-chemical series.
Not always an external dc power source is used.
On some structures only sacrifying anodes (zinc to protect steel) are installed.
Here you can find EXAMPLES OF IMPRESSED CURRENT CATHODIC PROTECTION DESIGN:
**broken link removed**
I wanna protect a big above ground structure, an Iron tower. I don't have well clear where to place the anode because the medium is not soil or water, it's air.
In your case the tower is coated. If it is not yet made, you specify galvanized coating of zinc for the material. If the tower is already made, there are specialized paints for this purpose.