Mix teff with water and let stand, covered, in a bowl at room temperature until bubbles and has turned sour, up to 3 days. It should be the consistency of pancake batter. Slowly stir in salt until it can barely be tasted.
Lightly oil skillet and heat over medium heat. Pour in enough batter to cover the bottom of a skillet. Cook briefly until holes form in the injera and the edges lift from the pan. Do not let brown.