Empty the mason jar into a (plastic) strainer, catch the milk kefir in a large bowl, pot, or pan.
After all the milk kefir has passed through the strainer, rinse out the empty jar with warm water and dry the jar with a clean tea towel on the outside of the jar.
Place the rinsed and dried jar on a kitchen scale and tare it so it's set to 0 g. Add back the correct amount of kefir grains (for 1 liter of milk kefir, you should add 100 grams of kefir grains). Once the kefir grains are back in the pot, add enough milk (1000 grams of milk will make about 1 liter of milk kefir).
After enough milk has been added, I close the jar with the airlock and place it back on the kitchen counter.
After about 48 hours, repeat this recipe for a new batch of milk kefir!