1. Make the corn masa: Put 1 cup masa harina into a bowl and make a well in center. Add 2⁄3 cup plus 1 tbsp. water to well and stir to form a dough.
2. Knead dough until it's soft but not sticky, achieving a consistency like that of Play-doh. Pinch off a palm-size piece and roll it into a ball.
3. Cut two 7" circles out of a cleaned plastic shopping bag. Put 1 circle on a tortilla press. Arrange dough in center; top with second circle.
4. Push handle down on tortilla press to flatten the dough. For a thinner tortilla, shimmy the handle side to side while pressing down.
5. Open the press, remove the top piece of plastic, and transfer the tortilla to the palm of your hand, pulling off the second piece of plastic.
6. Cook the tortilla immediately in a cast-iron skillet over medium heat, flipping occasionally, until lightly toasted. Repeat with the remaining dough.