Continue to cook until all the water has evaporated. This may take 1 to 2 hours depending on the amount of fats and skins you have. Once the water has gone, you will start to hear a crackling sound; a sound from the skins cooking. Stir the skins so that they don't stick to the pan and continue cooking until the skin have turn brown. Turn off the heat and let the fat cool. Strain it if there are impurities, keep in jar and freeze.