Combine the masa harina, water and salt, kneading in a revolving motion with your hands. Knead for a couple of minutes, until dough is smooth and has no lumps. If it feels too dry, add a bit more water.
Divide the dough into 12 balls, each about 2-inches in diameter. Line the bottom of a tortilla press with circles cut from a thin plastic bag (like the ones from the produce section of your grocery store). One at a time, place a ball of dough onto the plastic lining the bottom of the tortilla press, and top with another layer of plastic. Press down to make a flat disk as thick as a pancake, about 1/4-inch thick (much thicker than a tortilla). You can also flatten and form them by hand. Repeat with all 12 balls.
As you make them, place each sope on the hot comal or skillet. Let them cook about one to two minutes on each side, until opaque and speckled, and they can be flipped without sticking.
Take them off the comal and place them on a chopping board. Using a kitchen towel to protect your fingers, make a rim around each sope by pressing and pinching with your fingers along the edges. Return them to the comal or skillet, and let them cook for one or two more minutes per side, until thoroughly cooked.
If eaten the same day, they may be kept wrapped in a clean kitchen towel. If not, wrap them in a kitchen towel or paper towel, and store inside a closed plastic bag in the refrigerator. They will keep for up to 3 days, afterwards they will turn too hard. Reheat on hot comal or skillet for a couple minutes before eating. They can also be frozen and kept for months.