Making probiotic milk at home is very easy if you can get the proper milk. The proper milk is the one that can actually nourish bacteria, so a culture can properly develop. Yogurt, buttermilk, and kefir are all fermented milk products with probiotic bacteria, the difference is the recipe, and the resulted bacteria cultures.
Fermented-Foods.com offers this artical on making Kefir and Yogurt at home: How To Make Probiotic Milk