There are two types available in the stores. One is for tamales, the other is for tortillas. The difference is the grind. The masa harina for tortillas is a fine flour, the tamale one is courser but not as course as corn meal. For authentic tortillas, you cannot use corn flour. Masa harina is made by taking the dried corn and soaking it in a water/lime solution (to read more about this, go here: What is Masa Harina?). For any other grain, just grind to a fine flour.
Heat a dry griddle or heavy skillet over medium-high heat.
In a large bowl, mix the ingredients with a sturdy spoon or your hands until the dough is smooth and forms a ball. The dough should be quite moist but hold its shape. Add a little more water or masa harina, if needed, to achieve the proper consistency.
Form the dough into 12 balls approximately 1 1/2 inches in diameter. Cover the balls with plastic wrap to keep them from drying out. If any of the balls do dry out before cooking, knead more water into them. Unlike the dough for flour tortillas, this dough can be reworked.
Place one ball of dough in a tortilla press between the two sheets of plastic that are sometimes sold with the tortillas press or two a 1 gallon freezer bag, cut into two sections. If you don't have a tortilla press, you can either roll each ball between two sheets of waxed paper or press between two sheets of waxed paper with a heavy, flat bottomed pot (my pressure canner worked great for this). Flatten to about 1/8 inch thick. Carefully pull the plastic from the tortilla and lay on the hot griddle or skillet.
Cook for 30 seconds, flip over and cook about another minute, flip and cook the first side an additional 30 seconds. Place the cooked tortillas between two cloth towels until all tortillas are cooked. Store as above (once cooled, place in a gallon Ziploc bag and store in fridge).