1. Combine masa harina, salt and 11/2 cups of the water in a large bowl and knead with your hands until a uniform dough forms, 1 to 2 minutes. The dough should be slightly moister than fresh Play-Doh but just stiff enough to form into a ball. If necessary, knead a little more water into the dough. Let stand, covered with plastic wrap, for 5 minutes. The dough will dry out as it sits, so keep it covered with plastic while you work.
2. For 51/2- to 6-inch tortillas: Pinch off 3 tbsp dough and form into a 11/2-inch ball (about the size of a golf ball). Form more balls as you press and cook them. For 4-inch tortillas: Pinch off 1 tbsp dough and form a 3/4-inch ball. Form more balls as you press and cook them.
3. Heat a comal or flat griddle over medium heat until hot, about 2 minutes. Press a ball of dough between plastic rounds in tortilla press to form a tortilla. If uneven, rotate tortilla 180 degrees and press again to desired size. Peel off 1 plastic round, then, holding tortilla over the edge of your palm, carefully peel off the other round so that tortilla is dangling from your palm. Transfer tortilla to griddle by letting the dangling edge touch it and slowly pulling your hand back as you lay the tortilla down on the griddle. This will take a little practice, but its better than flipping tortillas onto the griddle because they rarely end up lying flat.
4. Cook until edges lift just slightly from griddle, about 15 seconds. Turn over (you can lift the edge of the tortilla with a butter knife or spatula to help you, but then grab it with your fingers and flip it over). Cook until a few faint brown spots appear on the underside, about 45 seconds. Turn over again and cook until tortilla inflates slightly (this may not always happen) and small brown spots appear on the second side, another 45 seconds. Transfer tortilla to a cloth-lined tortilla basket to keep warm as you make more tortillas.