Are you culturing your own kefir or buying it in the store? If you are buying it in the store, it is not the same as real kefir because no one has ever been able to get consistant results so they use powdered cultures like in yogurt to get consistantcy. If that is what you are using, treat the kefir as though it was yogurt in soap making as it is very high in sugar.
If you culture your own kefir using kefir grains, once you have cultured/fermented to the point you like the product (most often between 2 and 3 days) strain out the cultures and place them in fresh milk. Take the kefir and measure out the amount you want to use and keep in a covered container and leave out for another 24 to 48 hours, put the rest in the refrigerator. You want to cover the kefir loosely because it will continue to ferment and if covered air tight the lid will either blow off or the container may explode if glass is used. I use a plastic measuring cup and plastic wrap. It will be thick, creamy when done.
It doesn't smell great when applied to the face but you can't beat it as a mask. Some people mix with papaya or pineapple for additional enzymes or avocado for moisturizing. The lactic acid is more than enough for me. You will have to experiment to see how long it needs to be left on your skin.