1 ½ cups whole wheat flour (a finely ground flour will give you the best results)
2 ½ cups white flour
1 ¼ tsp salt
6 tablespoons non-hydrogenated shortening, cut into smallish pieces
In a small saucepan, bring about 2 cups of water to a boil.
In a large bowl or food processor, whisk (or pulse) together the flours and salt until well combined. Add the shortening, and either break it up with your fingers, or pulse briefly in the food processor, until it is reduced to tiny bits. If you're using a food processor, turn the mixture out into a bowl at this point. Add the boiling water bit by bit, mixing first with a spoon and then your hands to distribute the water and work the dough together. Add just enough water until the dough just holds together (generally no more than 1 ½ cups). Knead the dough for a few minutes on a floured countertop until it becomes smooth. Don't overwork. Shape into a ball, place in a plastic bag, and let relax on the counter for half an hour.
After the dough has relaxed, heat a heavy skillet over high heat (as with pancakes, you want to make sure you're at full heat or else your first efforts won't be good -- aim for just-shy-of-smoking). Cut the dough into 18 equal pieces -- you can either do this by weight (the exact weight will vary, depending on the water needed, so weigh your dough and then divide by 18), or cut your dough ball into 6 wedges, and then each wedge into 3 pieces. Roughly shape each wedge into a ball.
You want to roll out each ball into a thin circle on a floured counter top, as thin as you can make it, with an 8-inch diameter. But you don't need to do this all at once -- you'll have time to roll as the tortillas are cooking. I like to give maybe 3 balls a rough pass first, then let them relax a moment, then roll them to full size one by one as I'm cooking. Cover any dough you're not working with with a dish towel so that it doesn't dry out. As each tortilla is shaped, toss it onto your hot skillet. It will puff up, sometimes dramatically. When it is lightly puffed, turn it over and cook the other side. This will take just half a minute or so per side, depending on whether you like light golden spots, or nearly-burnt spots (I tend to favor the latter). Stack the finished tortillas on top of each other, one by one, on a rack or plate. Keep the whole pile covered with a dish towel so that they stay soft and pliable. Leftovers can be refrigerated in a sealed bag, and warmed in a low oven, or heated briefly over a gas flame.