There are several methods for this, but I will only write about one in detail, as that's the only one I have tried, and can confirm that it works, and works well.
First, buy a little pot of buttermilk from your supermarket of choice - I have Asda and Waitrose within walking distance; Asda does not stock it, Waitrose does. The price is about 50p. If you haven't got any milk in, buy some of that - whole, please, and not UHT (ultrapasteurised). If you have a way of obtaining raw milk, even better.... But don't worry if you don't. The process of culturing is supposed to make even the pasteurised, homogenised supermarket milk digestible.
Next, pour about a litre of milk into a glass container (something like a large Mason or Kilner jar is best - in fact, if you are planning to do a lot of this fermenting lark, investing in a few of those will be something you'll be very grateful down the line). Add the contents of your buttermilk pot. Stir well, cover, and keep at room temperature until the milk thickens and starts to curdle - 3 or 4 days usually, depending on the temperature. Once it does, put about half a cup into a smaller glass jar and store in the fridge to use in your next batch. Refrigerate the rest.
So, your litre of milk and a small pot of buttermilk has now become a litre of buttermilk. At this stage, feel free to use it in any recipes that call for buttermilk - soda bread, pancakes, whatever - or just drink as is. But if you are after whey, or planning to lacto-ferment your vegetables, keep your greedy paws to yourself and leave well alone. Your patience will be rewarded, I promise.
Right. You have a litre of buttermilk in your fridge, and half a cup of starter for the next batch - so get another litre of milk, and repeat the whole process. The initial batch of buttermilk will keep in the fridge whilst you do.
Of course, there is nothing to stop you buying two pots of buttermilk at once and doing the whole double batch in one single go.... I am just trying to save you 50p, especially as I just encouraged you to spend a fair bit more than that on sparkling new jars. If you want to be properly sensible and frugal, you'll ignore me anyway and use old coffee or pickle jars, and good for you.
Okay now, a week has gone by, you have two litres of buttermilk, you are fast running out of jars and fridge space (get used to it), but hey, at least your kitchen counters are clear again (don't get used to it) - now what ?
Now take your buttermilk out of the fridge and leave it on the counter. A day, two days, maybe three - this time you are waiting for the curdling proper; when the curds and whey are starting to visibly separate, you are ready for the next step.
The next step requires a large bowl, a strainer to set over it, and a muslin cloth to line the strainer. Got it all ? Good. Now pour your buttermilk into the strainer. Cover, and let stand for a few hours. After a few hours, tie the corners of the cloth into a knot and stick a wooden spoon through the knot. Rest the spoon over the top of a large jug or saucepan and let drip further. Do not squeeze. The liquid on the bottom of the bowl you initially used, and the liquid on the bottom of this new jug/saucepan is whey. When your muslin parcel stops dripping, put all your whey into a glass jar, cover and store in the fridge - it will keep there for up to 6 months.
What's left in your muslin parcel is the by-product - a delicious cream cheese. This will keep, covered, in the fridge for up to a month. You can use it as is, or add flavourings - I like adding at least sea salt crystals, perhaps freshly ground black pepper. Fresh or dried herbs are also good, chilli is lovely if you like it hot, and garlic is seriously yummy. Feel free to experiment and report back on your findings :o)
And there you go - as so much in the ways of a nourishing traditions kitchen, it could not be simpler - but it does take time and patience. And that, I feel, is as it should be - some things are worth waiting for.
Tomorrow: how to use your whey in lacto-fermentation of vegetables.