How is the process of Hagala is done? When can I use it? (General use, not Pesach)

Hagala is the process of kashering a pot or other utensil from an issur (= a forbidden food) that it absorbed while in contact with a hot liquid in a vessel that was or had been on the fire. After waiting 24 hours after the absorption of the issur, the cleaned utensil undergoing hagala is totally immersed (usually in a net bag) into boiling bubbling water on the fire. It is then removed and dipped into cold water. When kashering a large pot that practically speaking cannot be immersed because there is no larger pot available, the pot to be kashered after 24 hours is cleaned, and then filled to the brim with water that is brought to a bubbling boil until the water overflows the pot. Hagala is not sufficient to kasher a vessel that absorbed an issur on the fire without a liquid, for example if non-kosher meat were roasted in a roasting pan, the roasting pan could not be koshered through hagala.