Whisk the masa harina and salt together in a large bowl. Pour in 1½ cups of warm water and stir until a shaggy dough forms, then knead by hand for about 3 minutes until it's smooth and no dry masa is left. It should feel like soft Play-Doh. If it cracks when you press it, work in more water a tablespoon at a time. If it clings to your skin, dust in a little more masa harina.
Portion the dough into 12 balls, set them on a plate, and drape a damp cloth over them so they stay moist while you work.
Heat a nonstick skillet or comal over medium-high until it's good and hot. While it heats, trim the seams off a large zip-top bag to get two flat sheets of plastic, or cut two squares of parchment.
Sandwich a dough ball between the two sheets on the tortilla press and flatten it to about 5 to 6 inches across. Peel the plastic away and lift the tortilla into your hand.
Lay it on the hot skillet. Give it 15 to 20 seconds, flip, another 15 to 20 seconds, then flip once more and cook 30 seconds a side. It should puff on that last flip. Repeat with the remaining dough.
Move the cooked tortillas into a tortilla warmer as you go, or wrap them in a clean kitchen towel and slip them into a large zip-top bag so they stay soft.
Serve right away, or cool and store wrapped in the fridge for up to 5 days.
Notes
Storage and reheating. Let the tortillas cool completely before you put them away, and make sure they aren't damp, since trapped moisture makes them go bad faster. Wrap them in a clean kitchen towel, seal them in a bag, and keep them in the fridge for up to 5 days. To reheat, warm them on a hot skillet or comal for 20 to 30 seconds a side, or right over an open flame. Don't microwave them; they dry out and crack.
Freezing. They keep for up to 3 months in the freezer. Seal them in a freezer bag with as much air pressed out as you can and label it with the date. I like to put a piece of parchment between each one so they don't stick together, though if you let them thaw all the way they'll separate fine either way. Thaw overnight in the fridge before reheating.
Water. How much water depends on your brand. With Masienda I use closer to 1¾ to 2 cups, and with Maseca 1½ cups is usually right. Start at 1½ and go from there.
Thumbprint test. Roll a piece of dough into a ball and press your thumb into the middle. If the edges crack, it's too dry. The dough will often feel fine and still fail this, so trust the crack over the feel.
Don't have a tortilla press? Press the dough between two sheets of parchment using a large glass baking dish. The trick is pressing as evenly as you can so the thickness stays the same all the way around.