|Wash grains in a plastic sieve|
You can buy the "grains" online, they are a culture of bacteria and yeast that have been used for thousands of years to ferment a whole range of milks, kvass is fermented mare's milk. The shelf life is up to thirty days, its good on breakfast cereals and as a drink. Just don't expect to get inebriated the alcohol content is only 1 - 2%.
If you buy online your grains, a very small amount, will arrive in a plastic bag. Tip them into a plastic sieve, never metal, and wash with cold water.
|Add milk and put in a dark cupboard|
Pass the fermented mixture through the sieve again, store or drink the liquid and keep the grains for your next batch.......simples!
If you happen to have a bag made from a goat's stomach you can put the milk in that and hang it by the door where it will be agitated from time to time by people coming in and out as they do in the Caucasus.