If you can find it in the library, the book "Brain Maker" [0] has a good starting point in its recipes section. Here's [1] a video showing how it's done. You've got to buy the Kefir grains (or get them from someone you know who makes Kefir). Once you have the grains, you most likely won't need to buy them again, as you'll keep them alive and growing on your own. (Note for the grain-free folks, Kefir grains aren't actually grains. They're a bacterial culture conceptually similar to what you'd use to make yogurt.)

Basically, you combine water, Kefir, and sugar, then you wait a few days. Then, you drink. Or if you want to add flavoring (which I highly recommend), you do a second fermentation process: add in the flavoring (e.g. dried strawberries or something), bottle it for a 1-3 more more days, then drink or refrigerate.

[0] https://www.drperlmutter.com/about/brain-maker-by-david-perl... [1] https://www.youtube.com/watch?v=Y_d7wQ-lkvI