Make sure your work area and equipment is clean before you start making cheese.
Take your milk directly from the fridge (it needs to be cold) and pour it into the instant pot.
Mix together ½ cup of apple cider vinegar and 2 cups of filtered water. Add the mixture to the pot of milk while stirring.
Press the sous vide button and adjust the temperature to 90°F (see notes). Press Start (the time doesn't matter as long as it's at least 30 minutes).
While the pot is warming up, mix a ¼ teaspoon of rennet with a ¼ cup of filtered water. Set aside.
Once the pot has reached 90°F, pour in the rennet while stirring for about 15-30 seconds, until you notice the milk starting to thicken.
Put the lid on the instant pot, press down the venting knob and let the pot sit for 15 minutes or until there is a clean break between the curds and whey (see notes).
Once there is a clean break, use a long knife to cut the curds into a grid pattern. The curds should be around an inch thick.
Let the curds sit for another 5-10 minutes.
Next, use a slotted spoon to scoop the curds into a colander (make sure there is a dish underneath to catch excess whey). Use the spoon or clean hands to press out the whey.
Store the whey in the fridge for later use.
Fill the inner pot halfway with filtered water. Press the sous vide button and adjust the temperature to 170°F. Press Start (the time doesn't matter as long as it's at least 10 minutes).
Add the curds into the hot water and let them sit for 3-5 minutes.
Turn off the Instant Pot. Remove the curds from the hot water (rubber gloves are recommended here) and add salt to the cheese if desired.
Stretch and fold the cheese as you would if you were stretching and folding dough. Before it completely cools down and begins to harden, form it into a ball. You can place it back in the hot water if you need to stretch it more.
Place the mozzarella ball into an ice bath to cool it down quickly. Enjoy it fresh or store in the refrigerator in a sealed container for up to several days.