Start by emptying 1 kg of dry couscous (do not use instant couscous) into a very large shallow bowl. (The "bowl" I'm using here is called a gsaa.) Adjust the amount of oil and water in each step if using a different quantity of couscous.
Add about 1/4 cup of vegetable oil. Toss, rub and stir the couscous with your hands to distribute the oil. The oil helps prevent clumping, allowing each grain of couscous to remain separate as it steams.
Note: Some Moroccan cooks add the oil after the couscous has steamed the first time, but I prefer to add it initially.