This cheese is quite easy and rather quick - it takes less than an hour, including clean-up.
You'll need:
- NON-ultra-pasteurized whole milk (you can use 1% or something but it has much less fat, so it will make less cheese)
- citric acid
- rennet (tablet or liquid)
- water
- ice water (recommended)

Equipment:
- stainless steel or other non-aluminum 1-gallon or larger pan (aluminum will react with the milk)
- slotted spoon
- cheesecloth or butter muslin (butter muslin is just finer mesh than regular cheesecloth)
- other 3-quart (approx.) pan (this one can be aluminum)
- colander that fits into the 3-quart pan most of the way
- measuring cups and 1/2-teaspoon measure
- thermometer
- long knife that reaches to the bottom of your pan - I use a bread knife

VERY IMPORTANT: DO NOT use ULTRA-pasteurized milk. There are not enough natural cultures in the milk to create curd. You will end up with a ricotta-style cheese rather than a mozzarella. You do not need raw milk - regularly pasteurized milk is fine - but ultra-pasteurization is no good. I use Garelick/Dairy Pure (whatever it's called now) and it works great.

The milk companies pasteurize milk before sending it to stores. Regular pasteurization is at 145 degrees F for 30 minutes. Milk may also be pasteurized at 161 F for 15 seconds. Some companies, however, ultra-pasteurize at 280 F for only 2 seconds, which kills many beneficial bacteria.

IMPORTANT: Also, do not use an aluminum pan. The pan reacts with the milk and the bacteria and bad things will ensue.

Step 1: Dissolve Citric Acid and Rennet

Cut 1/4 tablet of rennet (or use 1/4 teaspoon of liquid rennet) and dissolve it in 1 cup of cool, non-chlorinated water. Chlorine will harm the enzymes. Stir to dissolve.
Stir 1.5 teaspoons of citric acid into 1/2 cup of water.

Note: The actual amounts of water are unimportant and need not be exact - they are merely so as to get the substances to mix better with the milk. So if you accidentally use 3/4 cup instead of 1 cup, it's not a problem.

Step 2: Heating the Milk

Pour the milk in the pan.
Stir in the citric acid now. Pour slowly.
Heat the milk in the 1-gallon pan to 90 degrees (F). Stir the whole time so it doesn't burn on the bottom.
Meanwhile, make sure the rennet is dissolved.

Step 3: Coagulation!!!

Once the milk reaches 90 degrees F, take the pot off the heat. The goal now is to keep the milk at 90 while the rennet does its work.

Add the rennet solution and stir well. It's ok if your milk is a little clumpy or grainy.

Cover the pot and do not disturb it for 5-10 minutes. I usually let mine sit 10 because it gives a firmer curd. The citric acid has created a comfortably acidic environment in which the rennet 'likes' to curdle the milk.

You will know your curd is ready when the milk is about yogurt thickness and can be pulled about 1/2 inch away from the edge of the pan.

If it's not ready after 10 minutes, don't worry. Give it another few minutes. If after 25 minutes it isn't ready, there's probably something else wrong:
- you used ultra-pasteurized milk (most common problem!)
- your rennet is too old
- you killed the rennet by heating it too high
- the milk is too cold

Step 4: Cutting the Cheese

OK, so this is actually called cutting the curd. But essentially, you're cutting the cheese. This is my favorite part, by the way.

Hold the knife at an angle more than 45 degrees and less than 90 degrees to the surface of the milk. Slice a 1/2 inch grid, as vertical as possible, all the way into the curd. You have created many square-ish vertical prisms. If you want, you can now retrace those lines with the knife tilted 45 degrees to the side, so that those prisms are sliced into pieces diagonally. You don't have to do this, though, because the curds will inevitably get broken up in the next step anyways.

Step 5: Cooking the Curd

Put the pot back on the heat and heat the curds to 110 Fahrenheit. This coaxes the whey out of the curd pieces. Stir the curds as carefully as you can. You don't want to break up the curds, but as I said before, it will happen inevitably. Don't worry about it.

Take the other pot at this point and fill it about 2/3 full of water. Put it on the stove also and heat it to 185 F.

Put the colander in the sink and line it with butter muslin (to catch the small pieces of curd). If you don't want to lose your whey (groan), put it in another pan or bowl or something to catch the whey.

Keep stirring...

When the curds are at 110 F, turn off the burner and take the pot off. Keep stirring the cheese for about 3-5 minutes. This also removes whey from the curds.

Step 6: Stretching

Now pour the curds, and the whey, into the colander. Let the whey seep through - you only need the curds. Now flip over the mass of curds and peel away the cheesecloth/butter muslin.

Bring the colander and the pot over to the stove, where you have prepared the 185 F water. After peeling away the cheesecloth, dunk the cheese, in the colander, into the water. If you dip before removing the cheesecloth, you will probably never get your cheesecloth clean again. You need to get the curds to 135 F, so they stretch properly. If you prefer you may don the yellow rubber gloves to protect yourself from the hot water. I prefer to just take it like a man and use my bare hands.

In between hot water dunks, grab both ends of the curd and pull away. Supposedly it's like stretching taffy (I wouldn't know; I've never stretched taffy before). Anyways, keep dunking and stretching. When it stretches nicely, no rips, and it's a bit shiny, it's ready.

You can pull it into logs to make the traditional string cheese, or you can tie the strings into knots. Form into balls to make Bocconcini. Or simply make a large, awkwardly shaped mass of curd. Up to you and your imagination. Put it in a bowl of ice water to cool it down quickly. Or, just proceed quickly to the next step and eat warm.

Step 7: Eat

Eat. Nothing more to say. If you want, marinade in olive oil. Should keep for about a week, and the recipe makes about a pound. In general, a gallon of milk makes ~1 pound of hard cheese.

To get supplies and information:
New England Cheese Company
http://www.cheesemaking.com/

You can also make grilled pizza with your mozzarella - see the instructable!