Into your clean half-gallon Mason jar, pour your buttermilk. Then, pour the 1% milk over top. Seal jar with lid and shake vigorously for one minute.
Place jar on kitchen counter out of direct sunlight for 12-24 hours until thickened. When thickened, refrigerate. Your buttermilk is ready. Use within two weeks, reserving the last 1/2c to re-culture another batch. And another. And another.