Add the flour and shortening (or lard) to a large bowl. Use a pastry cutter (or your fingers) to work the fat into the flour - you want it incorporated completely. Combine the salt and water in a measuring cup and pour about 3/4 of the water over the flour/shortening mixture. Use a fork to stir to stir together until all of the dry ingredients are moistened, adding the rest of the water (and even a little more), if needed. Turn the dough onto your work surface and knead until smooth - it'll only take a minute or two. The dough shouldn't be very sticky, but if it is, add a tiny bit of flour to your hands so you can work with it.
Divide the dough into 12 equal pieces and roll each into a ball. Put them on a plate, cover with plastic wrap and let rest for at least 30 minutes. This rest time helps the dough relax so it'll be easier to roll.
Set a heavy skillet (I used cast iron) over medium to medium-high heat. Working with one piece of dough at a time, set on a lightly floured surface and roll into a 7-inch circle. Add only enough flour so you can work with the dough - it will be quite thin by the time it reaches 7 inches in diameter.
Transfer to the preheated pan - you should hear a sizzle when you place the dough in the pan and should see bubbles forming across the surface almost immediately. Cook for 30-45 seconds on the first side, then flip and cook 30-45 seconds on the other side. The tortilla should be golden brown and might have a few dark spots, but you don't want to overcook it or it will be crisp rather than pliable. Transfer to a plate and cover with a kitchen towel.
Repeat with the remaining pieces of dough, and continue stacking them under the towel. Serve warm, or let cool then wrap in plastic and keep at room temperature for a few days, or pop in the freezer. Reheat in the microwave wrapped in damp paper towels.
Makes 12 tortillas