@i_love_coffee
Yeah, you just mix maybe 1 cup of PLAIN, UNSWEETENED, as natural as you can get kefir with gallon of plain milk (you can go for the whole or reduced fat). It's not a strict recipe to follow, as the main purpose of this is just to get the bacteria work on the pasteurised milk. If you want it fast, you can mix more kefir. I mix around half of cup with half a gallon of milk. And let it stand on your kitchen counter top for day, two. Usually it works out fine, and depends on the room temperature; The result should be a thickened liquid with the kefir-like sour taste. The best thing is to mix it well, as during the process the thick part goes up, and at the bottom of the jar/crock you can see the plain buttermilk (it's a yellowish almost plain liquid, not like the buttermilk from store). Never have tried the kefir grains.
Lactose free version. Maybe it's worth to give it a shot and mix the 99% lactose free kefir with the lactose free milk, from the diary section, but I've never tried that and do not guarantee the results
Traditionally, kefir was done but putting aside fresh milk. Leaving it in the room temperature for a few days, so it can become sour and thick (don't think it's the same as spoiled milk). But it was before all that bacteria killing during the ultra-pasteurization process. If one used the whole milk, straight from cows, there was a pure cream on the top of the kefir, when it was done. People gathered that top layer, and used it as a cream or made a butter from that.
If you have a lot of kefir, you can even made a delicious home made cottage cheese from it.