Combine 250g masa harina with 330ml hot water to form a dough. Leave covered for 15 minutes. Knead in more water if necessary; the texture should be like soft clay but not sticky.
To form the tortillas, pinch off a piece of dough and roll into a ball. Have a little pot of cold water nearby to dip your fingers into when rolling if the dough feels dry. 15g balls make 8-10cm tortillas, 25-30g balls make 12-15cm tortillas.
Place the dough ball between two sheets of plastic (a plastic bag cut into 20cm squares works well), and roll out into a 3mm thick circle with a rolling pin or use a tortilla press. Peel off the top piece of plastic, then place the uncovered side of the tortilla in your palm and carefully remove the other piece of plastic.
Heat a frying pan (without any oil) to medium then gently place your tortilla in it, so as not to trap any air. Cook for 15 seconds, or until the tortilla releases itself from the pan, then turn over and cook the other side for 30-45 seconds. Turn once more: the tortilla should puff, but you can help it along by gently pressing the edges when you see a bubble forming. A cooked tortilla that has puffed has two faces: the puffed side will be thinner and have a few brown spots (that's the top) and the other, thicker, paler side that hasn’t puffed (the bottom).
Have a folded clean tea towel ready on the side. As you cook each tortilla tuck them into the tea towel to keep warm.