Kefir is a milk product a bit like yogurt, but with even more digestive benefits. You make kefir using grains (the "mother" bacteria and fungi), and since the grains double in size every month or two, you can often get starter cultures for free from a friend. But what do you do once the grains show up?
Rinse your kefir grains. Since the grains (the solid gob) have been sitting in the same milk during transit, they'll need to be rinsed. Place them in a colander and pour milk or unchlorinated water (such as well water) over the grains until they look clean. Discard the rinse liquid.
Place the grains in a glass jar and add one cup of whole milk per tablespoon of grains, then cover the top of the jar with a cloth napkin or other piece of breathable fabric. You can use a rubber band or the ring of a mason jar to keep the cloth in place. The cloth is just to prevent bugs and dirt from getting in your brewing liquid. Be sure to use whole milk (cow or goat is fine), and unpasteurized milk is even better if you're a raw-milk drinker.
Put the jar in a warm place, such as on top of your refrigerator, and leave it for 24 to 48 hours. You might actually have to wait 3 days the first time since the microorganisms will take a bit of time to recover from the trauma of being mailed. You'll know the kefir is ready when it solidifies and you see a bit of separation of whey (clear liquid) from the milk solids. If you want a mild-tasting kefir, stop after 24 hours. A more sour kefir with more beneficial fungi will develop when fermented longer.
Once your kefir is ready, remove the grains and place them in a new jar with another cup of milk to ferment the next batch. To remove the grains, you can either spoon out the contents of the jar into a bowl and scoop out the grains, or you can strain the kefir through a plastic or stainless-steel sieve. Do not allow anything other than plastic, glazed ceramics, and stainless steel to come in contact with your kefir grains or you will harm the microorganisms.
Eat your kefir. Your kefir grain(s) will have been stressed a bit by their trip, so be aware that your first batch of kefir might taste a bit funny. This is more likely if you're changing to another type of milk (such as goat milk). The strange taste should go away as the microorganisms adapt to their new surroundings, but you can discard the first batch(es) if necessary. Once the flavor evens out, you can eat your kefir any way you'd eat yogurt. Our favorite methods include mixing in honey and cocoa (combine warm honey and cocoa first, then add the kefir), mixing with applesauce, or making kefir cheesecake. You can also use kefir grains to make sour cream out of fresh cream and to make kefir out of coconut milk and other non-dairy milks. However, your grains will be happiest in whole milk, so try to give them a round or two of milk in between other substances.
As your kefir grains grow, you'll need to increase the amount of milk, or split them. A rule of thumb is to use a tablespoonful of grains per cup of milk. We like to culture our kefir for two days in the winter, so I keep two jars of kefir going at all times, one to eat today and one to eat tomorrow. Once you have more grains than you can handle, pass some on to a friend and keep the cycle moving!