In a mixing bowl, whisk together salt and masa harina. Then use a mixing spoon to mix in the water. When the dough gets too hard to mix, begin kneading the dough with your hands until everything is combined.
Form the dough into balls about 2 inches in diameter. Use your palm to flatten the balls into a disk.
When they are flat, lay about 1 tbsp of shredded cheese in the middle, then ball up the dough to form a ball again. Finally, flatten into a disk again now that the cheese is inside.
Heat a cast iron skillet over medium heat, and allow to heat up. Add the pupusa into the pan, and cook for about 5-10 minutes on one side, until the outside is crispy and you see a few small char marks. Flip the pupusa and grill again on the other side. Enjoy!