The bottom chamber of the moka pot needs to be filled with cold water, until it is under the valve, never go over the safety valve! The approx amount of water would be 150 grams.
The heat will make the water boil and the pressure will produce steam into the upper part of the Moka Pot. A gurgling sound will be the indicator that the coffee is done, you can now remove the Moka Pot from the stove.