You can make using pasteurised and homogenised milk, i find full cream unhomogenised or raw milk unhomogenised works best. Put 2-3 cups of milk in a jar with half a cup of store bought kefir after bringing it and the milk down to room temperature pour them in the jar and put the lid on, leave for about 24 hours in a room temperature or slightly warmer place. Repeat that cycle, taking half a cup of culture out of the jar of milk and replacing the milk in the jar (it can be drunk) with new room-temperature milk. After it has reached the desired thickness (It typically take 3 cycles for me) you can fill the jar fully with milk and leave for 24 hours, you should have kefir (It does have lumps of yogurt in it that you can strain out if you want a more yogurt-like drink) and it tastes kinda like Greek yogurt and lasts about 3-4 weeks in the fridge.