Soak grains for 6-8 hr. in a quart jar
Drain water, cover jar with a screened lid, and lay the jar on its side for the grain to spread out and be exposed to air. Cover with tea towel to keep light (and fruit flies, in summer) out of the aerating jar.
Rinse and drain the sprouting grain, twice a day, for 1-2 days. The grain does not need to grow a long tail to be sprouted.
Put grain and half of the water in a high-powered blender and pulse for about 1 min. until grain is broken down. Then pour the mixture into a half-gallon jar (or two quart jars) along with the remaining water. Cover the jar with a mesh lid, then cover the lid with a tea towel.
Allow the mixture to culture in a dark place for 24 hours or more. The longer the mixture ferments, the more tart it becomes. Do not be alarmed that the mixture does not smell particularly appealing! It should taste tart and lemony, reminiscent of the grain you have sprouted. If you feel the need to smell it, just rest assured it does not taste like it smells.
I have never had Rejuvelac mold. I have heard of it happening, though. If yours molds, throw it out and start again. This is more likely to happen when the fermented grain is neglected for too long or when your conditions are very hot and humid.
Cover the Rejuvelac and refrigerate it. It will last up to 1 week in the fridge. I use it with the sprouted grain as the base of green smoothies. However, if I drink it as a beverage, I strain out the grain solids with a nut milk bag or cheesecloth.
You can reuse the sprouted, fermented grain once more to make a second, weaker batch of Rejuvelac by adding water to it again and leaving it on the counter for another 1-2 days.
I recommend rotating the grains you use for the widest possible nutritional variety.