To your finished milk kefir, add lemon zest, vanilla extract and liquid lemon drop stevia.
Use an electric hand mixer to blend it all together right in the jar.
Next, place your flavored kefir on the countertop covered with a plastic lid for 2-4 hours and have it go through a second ferment.
You can enjoy immediately or refrigerate covered for up to 2 weeks. The longer it stays in the refrigerator, the more sour it will get.