Add the flour, baking powder, salt, and shortening to the bowl of a large food processor.
Pulse several times to cut the shortening into the flour, until the mixture resembles coarse pea-sized crumbs. This can also be done by hand using a pastry cutter.
Add the hot water to the flour mixture. It doesn't need to be boiling, but it should be very, very hot. You can either microwave it for a minute or two or just use your hottest tap water.
Pulse again until the dough comes together in a ball. It will make a fairly sticky dough.
Dump the tortilla dough onto a lightly floured surface and knead several times until smooth and elastic. You may need to sprinkle the dough and rub your hands with a little extra flour so you are able to work with it, but try not to add too much flour or the dough will become tough. It should stay fairly tacky.
Divide the dough into 16 even-sized balls. I find it easiest to do this using a pastry cutter to cut the ball of dough in half, then in halves again, and again, then rolling each piece of dough into a ball between the palms of my hands until there are 16 ball of dough that are roughly the same size. Cover them with a damp towel and let them rest for 15 minutes so the dough can relax.
Heat a large cast iron skillet over medium-high heat.
Working with one ball of dough at a time, press it into a disc between the palms of your hands and dip into a little extra flour, shaking off any excess. Keep remaining dough balls covered with the damp towel while you are not working with them.
Roll the dough very thin using a rolling pin, turning the dough frequently a quarter turn at a time to form a circle. It's okay if they come out misshapen - they will still taste just as good! I tend to roll out 4-5 tortillas at a time, cook them, then roll out the next group.
Lay each tortilla, one at a time, in the hot skillet, letting it cook for 20-30 seconds until it starts to bubble slightly and brown spots appear on the bottom. Then flip and cook for another 15-20 seconds on the other side.
Transfer the cooked tortilla to a plate lined with damp paper towels and cover it with another damp paper towel. Continue to cover and add tortillas as they are finished, recovering with the damp paper towel to keep the cooked tortillas warm, soft, and pliable, until all tortillas have been cooked.