Heat your large cast iron skillet over medium heat. Assemble your ingredients as the pan heats, because you won't want to leave it once you get started. Crumble up the sausage into small pieces. First you make a roux, melt the shortening in the skillet. Put in the flour, and stir constantly, adding more to get a thick roux, almost a paste. Keep stirring. If you make it too thick, add a little more shortening. Cook the flour until it is golden. Keep stirring. Then add the sausage. Keep stirring. when the sausage is cooked, that is no longer pink, then add enough milk to cover. Keep stirring. Reduce the heat a little once the milk starts to get hot. The gravy will start to come together in a minute or so. If it gets too thick, add more milk. Once its thickened and hot, add the pepper, salt, and hot sauce.