This information/procedure used to be given in the Fiat owner's manual and e.g. Haynes manuals. You measure the height from the centre of the lamps to the ground (there may be a small circle on the lens) and mark this on e.g. your garage door, you need a level surface and need to position the car perhaps 25ft. (c. 7m) back from the door, before making any adjustments set the load adjuster levers on the lamps to high. If you look at the dip beam pattern, you'll see a horizontal line which transitions to an upwards sloping( c. 30 degrees) line to the left(for rhd cars). This transition point needs to be set slightly low and to the left of the centres you marked earlier. I used to set lamps this way on older cars but its not much use on modern,complex lamps. As others have said its best to have the lamps set using a proper beamsetter to be sure of passing the MOT.
Al.