In a large bowl, mix the flour and salt together with a fork. Cut in the fat until it is the size of peas, then add the milk and sourdough.
Mix everything together well, then cover the bowl and set it in a warm place for 8 to 12 hours.
When you're ready to make tortillas, preheat a griddle or large skillet over medium-high heat.
Divide the dough into 12 to 16 pieces, depending on what size of tortillas you want.
Roll each piece of dough into a ball and place it back in the bowl, covering the bowl between.
Dust a clean work surface with flour. Roll each ball of dough into as thin a circle as you can without breaking it.
Carefully set one tortilla on the griddle and cook for 2 minutes. Flip and cook for another 2 to 3 minutes, then transfer the tortilla to a towel-lined basket.
Cook all of the tortillas, keeping the basket covered when you aren't using it.
Serve the tortillas warm or reheat them on a griddle. When you reheat them, sprinkle each one with a bit of water to keep it moist.