If using masa harina, place in a large, deep bowl and slowly add the water. Knead into a moist, cohesive dough that cleanly pulls away from the bowl and no longer feels sticky or thin, about 3 minutes. To test whether the masa is adequately hydrated, grab a piece, roll it into a ball, and flatten it. If the sides crack, the masa needs more water. The final texture should be slightly damp, like cold clay. Once the masa is ready, cover it with a damp dishcloth and let rest 15 minutes, which will result in a softer, more pliable dough.
If using fresh tortilla masa, sprinkle a little water, a tablespoon at a time, onto the masa and knead firmly until soft and pliable. You should need 1/4 cup water at the most, depending on how dry and crumbly the masa is.
Heat a comal or nonstick skillet to medium-low heat.
Break off a knob of masa and roll it into a smooth ball, about the size of a golf ball. Cover the rest of the dough with a damp dishcloth.
Flatten the dough ball between your palms, so it’s about 1/2 inch thick. Place on one side of a tortilla press lined with square sheets of plastic (these can be cut from plain grocery bags but avoid those with lettering or illustration as the coloring could leach onto the tortillas). Make sure the dough circle sits smoothly between the plastic sheets, then close the press. Push down the lever.
Open the press and rotate the tortilla a quarter-turn, keeping the sheets intact. Close and push down on the lever again. Continue to rotate the tortilla until it’s evenly pressed into a round about 1/8-inch thick. (If you have a heavy press that will make a thin, even tortilla every time, you can skip the rotating routine.)
Open the tortilla press and peel back the top plastic sheet. Place the tortilla face down in your open palm, so about half of it sits in the center of your hand, the other half hanging off. Gently peel back the remaining plastic sheet. The dough should not stick to the plastic or your hands. If it does, you’ve added too much water and need to start over.
Thicker tortillas will be easy to place on the comal. Tortillas made with fresh masa, however, take some practice. With half the tortilla resting in your open palm, and the other half hanging off the side, stand over the comal and slowly move your hand in a horizontal fashion, spreading the tortilla so the underside hits the comal first.
Flip—with a heatproof spatula or your calloused fingers— as soon as the outer edges of the tortilla start to darken and look less moist, about 45 seconds. The tortilla should have some dark-brown freckles at this point. If it doesn’t, raise the heat. Alternately, if it’s blackened, lower the heat.
Flip the tortilla again as soon as small air bubbles start to appear on the freckled surface, about 35 seconds. At this point, after the second flip, the tortilla might inflate. Cook the tortilla for another 25 seconds, and then push it to the side of the comal, away from hot direct heat, and cook 25 seconds more. Flip once more and cook for another 45 seconds, for a total of roughly 3 minutes per tortilla, depending on how hot your stove is. Place the tortilla in a dishcloth or covered basket to keep warm.
Repeat steps 3 through 10. Serve the tortillas warm and store any leftovers in an airtight container in the refrigerator for up to a week, or alternately, freeze for longer storage. To reheat, place thawed tortillas on a comal, or reheat individually on a gas burner.