In a small bowl, mix the rennet with 1/4 cup water to dissolve. In another small bowl, mix the citric acid with 1/4 cup water to dissolve.
Place a 5-quart pot into a large bowl with enough room to pour water around the sides. Pour your milk into the pot and add warm water to the bowl—you're creating a water bath to gently heat up the milk. Once it reaches 86°F, add the citric acid solution and give it gentle stir with a slotted spoon to evenly distribute. Allow the mixture to sit for about 10 minutes at 86°F.
After 10 minutes, add more warm water to the bowl to bring the temperature of the milk up to 90°F. Once the milk reaches 90°F, add the rennet mixture. Very gently stir the milk in an up and down motion to disperse the rennet for about 1 minute, taking care not to stir too vigorously. Allow the mixture to sit for about 30 minutes.
At this point, the curds will have come together into a solid-looking mass. Add more warm water to the bowl to bring the mixture up to 105°F and allow to sit for about 10 minutes.
After 10 minutes, it's time to drain the curds. Place a colander on top of a bowl. Using a slotted spoon, lift the curds and place them into the colander to let the whey drain off. Lift the colander to keep draining. Using your hands, very gently press the curds into the colander to release more whey. You want to remove as much whey as possible while handling the curds as gently as possible. They should feel firm when they're ready for kneading.
Add a few cups of hot water (we think 180°F is best) to a large bowl. Food-safe gloves can help protect your hands from the heat if you're sensitive.
Break up your drained curds into evenly-sized pieces. Gently drop the curds into the hot water. Use all the curds to make one large mozzarella ball, or divide them in half to make two smaller ones.
Using a large spoon, lift the curds to see if they're ready for kneading. They're ready when they are melty and stretching off the spoon.
Now it's time to stretch and knead, salting your cheese as you go. If the cheese starts to get cold and stiff, dunk it back into the hot water. Keep stretching, kneading, and dunking until the cheese starts to feel smooth. This can take anywhere from 5-20 minutes.
Form your cheese into a ball, or any other shape you'd like.