Mix the water and sourdough starter discard together in a small bowl and set aside.
In a medium mixing bowl, whisk the flour, salt and baking powder. Add the vegetable shortening and use your hands to work it into the flour mixture. (It will have a fine, sandy consistency.)
Pour the water/sourdough starter mixture into the dry ingredients and combine with your hands until there are no dry bits left in the bowl. Cover and let rest for 30 minutes on the counter. (The dough will be stiff.)
Divide the dough into 16 pieces and roll each one into a ball. Place the dough balls on a parchment lined baking sheet, cover with a towel, and allow the dough to relax for 30 minutes. (Alternately, you can place the dough in the fridge, covered, to ferment until you are ready to cook them.)
Preheat a cast iron skillet over a medium-low heat until it is very hot. Working with one dough ball at a time, use a rolling pin to roll it into a thin round, about 6" in diameter. Use flour as needed to prevent sticking.
Carefully lay the dough on the hot skillet and allow it to cook until bubbles start to form on top and the bottom is slightly charred. Flip it over and continue cooking on the other side until cooked through.
Repeat with the remaining dough balls, keeping cooked tortillas wrapped in a kitchen towel to stay warm until ready to serve.