Mix masa harina, salt, and 1/2 cup plus 2 tablespoons water together in a bowl using your fingertips until dough forms and pulls away from the sides, 2 or 3 minutes. Form into a firm, flattened ball and divide into 8 equal portions. Roll each portion into a ball. Keep dough balls covered with a damp paper towel as you fry them, one at a time.
Place dough ball between 2 sheets of plastic or a split open zip top bag. Flatten to form a small circle about 1/8 to 3/16 inch thick.
Heat oil in a skillet over medium-high heat. When oil is hot, slide in the first circle. Very carefully baste hot oil over the top of the shell with a spoon to keep oil circulating over the top surface. When shell has puffed (after about 15 seconds), cook another few seconds and flip it over and cook another 30 seconds. Remove with a slotted spoon and drain on paper towels. Repeat for each shell.
When shells are still warm and pliable, gently push down puffed tops to create room for the fillings and slightly crease the shells for folding.
Fill with your favorite taco fixings and serve while shells are still warm.