There’s the easy way to flavor milk kefir, and then there’s the really easy way. The easy way consists of secondary fermentation. You basically strain your grains out as usual, then chop up whatever berries, fruits, spices or other flavorings you want to use, and put them into the strained kefir. Let it sit for another round (up to 24 hours), strain out the fruit (or not) and enjoy your flavored kefir! You may want to add a bit of sweetener to taste if that’s your bag.
The second way to flavor kefir is just to throw fruit or ginger juice or cinnamon or whatever sounds delicious to you into your strained kefir and blend it all together (I usually go hand blender when using this method). This is preferable for anything highly acidic, such as citrus fruit or juice, which will make your kefir too acidified in secondary fermentation. Again, if you want sweetener, just add your favorite kind before blending!
For me, the whole point of making my own kefir is customization. I can make rosemary ginger kefir with whole, organic milk from a local farm and happy cows. This destroys the idea of some filler, hormone and chemical-laden grocery store product that I can only get in strawberry, mango or pomegranate açai flavor!