Bring a pot of water on the stove to a boil.
Insert the glass jar horizontally into the boiling water to allow the water to enter the jar. Remove the pot from the stove after one to two minutes and allow the water to cool. Pull the sterilized jar from the water and dry with a clean, dry cloth.
Fill the jar half-way to two-thirds full with purified or distilled water.
Add the chlorella culture to the water in the jar with a spoon. Only use certified chlorella cultures from reputable sellers and follow the manufacturer's directions.
Place the jar indoors in a sunny location. Attach an aquarium thermometer to the jar to monitor the temperature of the water. Follow the manufacturer's instructions to attach the thermometer. Some thermometers stick to the outside of the jar, while you must insert others into the water for an accurate reading.
Maintain a constant water temperature of 80 to 85 degrees Fahrenheit. Use a heat lamp if necessary to heat the water, and move the jar away from direct sunlight to cool the water when needed.
Harvest the chlorella after a week of growth.