Set up a tortilla press (or a heavy skillet for pressing) with two squares of plastic or a cut-open zip-top bag. Have a clean towel ready for stacking cooked tortillas.
In a mixing bowl, whisk together the masa harina and fine sea salt. Pour in the warm water and mix with your hand until a soft dough forms, then knead in the bowl until smooth and pliable, 1–2 minutes. The dough should feel like soft modeling clay and not stick to your hands.
Cover the bowl with a damp towel and let the dough rest to fully hydrate, 20 minutes.
Preheat a dry cast-iron skillet or comal over medium-high heat until hot, about 5 minutes.
Divide the dough into 12 equal portions (about golf-ball size) and keep them covered with the damp towel so they don’t dry out.
Line the press with plastic, place one dough ball in the center, and press firmly to a 5–6 inch round of even thickness. If using a skillet to press, flatten between the plastic using firm, even pressure.
Peel the tortilla from the plastic and lay it onto the hot skillet. Cook until the edges look dry and the underside releases easily with light brown spots, 45–60 seconds.
Flip and cook the second side until more spots appear and the surface looks set, 30–45 seconds.
Flip once more and cook 20–30 seconds, pressing gently with a spatula to encourage puffing; remove when lightly speckled and pliable. Stack in the towel to keep warm and steam.
Repeat pressing and cooking with the remaining dough, adjusting heat so tortillas spot but do not char. Serve warm.