Heat a pot of water until very hot (160 degrees F to 170 degrees F). Add a generous amount of salt. Place a large bowl filled with ice water nearby. Put the curd in another large bowl; add enough hot water to cover the curd.
Stretch the cheese:
Let the curd soften, about 3 minutes, then gather it into a ball and hold it over the water; fold and pull it like taffy until elastic, about 5 minutes, dipping it back into the hot water if it gets stiff. Don't overwork the curd, as this will make your finished cheese rubbery. If the water cools too much, simply pour it off and add more of the hot water. When soft and smooth, divide the curd into 4 portions and form each into a ball.
Let it cool:
Plunge the mozzarella balls into the bowl of ice water and let cool.
Slice and eat!
Serve the fresh cheese at room temperature. Refrigerate leftovers, covered with salted water, for up to 1 week.
You'll need fresh cheese curd to make mozzarella. Order it online or ask a store that makes cheese to sell you some.