John S said
I have never made it, but you've got me interested.  I've heard of it, but I'm not going to pretend that I'm sure I can do it right.  
Sounds like it would be fun to try.
John S
PDX OR

OK let me know if you want to go for it, I can share some of my kefir "grains" to get you started.  There are a lot of videos online about the process. Be sure to search for "water kefir".  There are two kinds of kefir - water and milk.  The milk process uses different grains and is a totally different process.

You can also buy the grains online, either dehydrated or active.  But it takes a while for them to wake up after being dormant. If you get grains from a friend, they will be active right away.  I got mine from a seller in Minnesota, on Etsy.  I started about a month ago with 1 tablespoon, now I have about 1/2 cup of grains.

I am finding that the grains LOVE apple cider.  i.e. they grow fast and produce a lot of bubbles when I add cider.

The microbes seem to be able to penetrate the fruit/berries, thus extracting the flavors without me having to mash them up first.  That is a huge selling point for me.  e.g. just drop in a handful of cornelian cherries, and they flavor the kefir.  Then strain them out when the fermentation completes.  As I mentioned you can then either eat the "carbonated" berries/fruit, or compost them. Although the process moves some of the flavors out of the fruit, and also removes a lot of the sugars.  Sugars are the food for the grains.

One area I'm still figuring out is how to get a good carbonation without producing alcohol. If you tried hard I think you could make it up to 2% alcohol, but I don't want any.  The alcohol production seems to occur after you seal the bottle, if there are still a lot of sugars present.  But you need to seal the bottle to get some carbonation.  I am sure I will figure it out.