Mix salt into the masa harina and then gradually add the hot water to the dough until you have a consistency is firm and springy. If it is too dry simply add more water. If you've added too much water and the dough becomes sticky the add a dash more flour. Cover and leave to rest, although I've used it immediately when time was short.
Preheat a non stick pan or griddle and then using a flat surface separate the dough into 3 inch balls. Flatten between two pieces of greaseproof/waxed paper, and roll till it is a suitable thickness. Or if you're flash flatten it in your tortilla press...check my pictures and you'll know I'm not an owner of a press. Heat it in your pan or griddle until it looks like you like your tortillas. I like mine a little crispy. Flip, repeat and eat with your filling/topping of choice.