Fill a half gallon container such as a plastic milk bottle. Remove any paper or plastic label on the outside of the container. Fill it at least partially with sand, or gravel. Then add water if more weight is needed. If you just fill it with water, though, the container will move around in the tank and interfere with the mechanism.
Seal it shut and place the container in the tank of the toilet. Carefully lower it into the water and replace the lid of the tank. You’re all done!
A sealed half gallon container will save half a gallon on each flush. If you, like most people with regular toilets, flush 5 times daily, with this simple process, you will save 350 gallons of water every month.