Put ½ tsp kefir grains in the jar. Add the milk, leaving about 2cm head room if using a clip top jar, or at least 5cm for a cloth-covered jar.
Set aside at room temperature for 18-24 hrs to ferment. It’s turned to kefir when the milk has thickened. It may have set and separated, with pockets of whey forming – this is quite normal.
If you can’t strain it straight away, put it in the fridge to stop it fermenting further, the flavour can get quite strong – you can strain it anytime over the next 48 hrs.
Strain the kefir through the sieve or straining funnel into the jug or bottle. The grains are quite robust and will withstand gentle stirring.
You can drink it straight away, flavour and refrigerate it (a slice of lemon peel or a drop of lemon oil add a delicious fresh taste), or leave it at room temperature for a few hours to make it taste stronger. Storing it in the fridge will slow down the fermentation by the microbes and it should remain pleasantly useable for 7-10 days.
In order to make some more kefir, rinse out the jar, return the grains (don’t wash them, there’s no need), and start again from the beginning.