Add kefir grains to a glass jar of unpasteurised organic milk. Close the lid but make sure that some air can still get through. Kilner jars without the rubber ring are perfect for this. Once the milk has fermented, it will start to thicken.
Leave until milk thickens. This will depend on how many kefir grains do you have and on the air temperature. The more grains you have, the quicker the process will be. You speed up the process by placing the jar next to a radiator or onto a heated mat. These mats are usually called Reptile Vivarium or Propagator Heat Mats and are available on Amazon. I tend to use a 1 littre kilner jar with about a hand full of kefir grains and this takes about a day to turn into kefir.
You can check the milk, by giving it a stir after about a day. Kefir can thicken at the bottom but not at the top. In this case, I usually stir the whole thing and get a nice runny consistency. Alternatively, you can leave it till it starts to separate a bit into whey and curds. The layer of yellow liquid is the whey. At this stage, kefir is usually a bit stronger (sour) tasting as more milk sugars are converted into good bacteria. When testing with a spoon, kefir on top will look like set yoghurt.
Once you are satisfied with the stage of fermentation that your kefir is at, you can strain it through a colander. Use a spoon to crape the bottom of the colander, which will help the kefir to drip through.
Once kefir grains are the only thing that are left in the colander, you can add them back to the empty jar that you just made your kefir in. You don't even have to wash the jar that you made your first batch in. The kefir that is left there will speed up the process of second batch.
Top the jar with kefir grains with raw organic milk, which will soon give you a new supply of kefir.
You can drink your kefir strait away or store it in a fridge. Its great in smoothies and lassies. You can even make cottage cheese from it and add it to your baby food.