Put the grains and the milk in a non-reactive container you can wash easily - a glass jar usually works well.

Cover with light fabric or a paper towel, held in place by a rubber band or something of the sort. You want air to go through but not dust or insects.

Leave at room temperature for approximately 24 hours. Check to see if it is ready, give it a little more time if not.

When the milk has thickened, strain it away from the grains. The grains may now be reused - the liquid is milk kefir.