Cover the jar with a cloth secured with a rubber band and let sit in a warm place 24-26°C (75-80°F) for 48 hours.During summer, I leave the jars in a closed cupboard. Light does not affect the grains, so if you decide to leave the jar on the counter, it's okay too.
In winter, you may place the jar on top of your fridge. It is usually a nice warm spot to ferment your grains.
If fermentation conditions are too cold your grains will grow slowly or not at all.